Pros: Good spatial imaging for an IEM; good sound; very comfortable and easy to put on
Cons: barely isolates at all (!); line noise (!); chord too short; even the case is designed poorly
Let me start with the good, and then explain why you should never buy this IEM.
Sound signature: The treble, mids and bass are sort of like that of an HD800, except downgraded. For example, one of the songs I use to test headphones is Michael Jackson's The Way You Make Me Feel. One of the key parts I listen for is that initial drum blast. On this, it sounds very close to the way it sounds on the HD800, except it's just missing a little bit. Moving on to a treble heavy song, it sounds similar to the HD800, but slightly muddier and the highs seem a little rolled off. Something is missing with the mids/voices as well -- they seem a little bit more distant. In general, it sounds like the HD800 but it's missing a little of that oomph, exhilaration or spine tingle that the HD800 gives (and a little is a lot when you're comparing headphones in this price range). Despite the fact that I keep noting differences between the IE800 and HD800, I do definitely sense a similar sound signature. For example, like the HD800, voices, instruments and all sound "natural" (they way they would sound right in front of you) as opposed to that synthetic and adulterated sound you hear in so many headphones. Well this is the best I can do now but when I can better articulate the similarities and differences I'll edit. But please do not misunderstand, this is a downgraded HD800, not at all an HD800. They're similar, yet also quite different.
Comparisons with Shure SE846: Is the bass as good as with the Shure Se846? No. Shure has invented a magical device in the SE846 that delivers beautiful, clean, tight and undistorted bass of whatever degree you want (depending on the filter you use). This is pretty good, not nonexistent bass (like, say, the Shure SE425), but I would compare it to something between the white and blue SE846 filters and even then the SE846 might be very slightly better in terms of quality. Is the treble as sparkly as the Se846 with a white filter? I'm not even sure about that; I would have to think about it a bit and do careful tests. But I disagree with reviewers I've seen that claim the Se846 sounds like its treble was rolled off relative to IE800. The IE800 has OK treble, but it also feels slightly rolled off relative to sparkly treble kings (like the HD800 and Audeze LCD XC).
Spatial imaging: For an IEM, the spatial imaging on this is amazing, better than the imaging of the Shure SE846 (but it's still no where near the spatial imaging of the HD800, and other over the ear cans can also do better; ultimately this is still just an IEM). It achieves that effect of making certain things feel closer and certain things feel farther away. On some songs you get a version of that cool echo/spaced out effect you get on the HD800, which is impressive in an IEM.
Detail and instrument separation: Detail and instrument separation is very good. Again this is not as crisp sounding as the HD800, but it does have a lot of detail. It's quite good. Of course, the cool aspect of the HD800's detail/separation is that the detail is placed in a certain virtual location, and as noted above this can't do that as well as the HD800.
Comfort: You do not need to go through an ordeal to insert these, unlike the Shures mentioned above. In addition, the wires are not stiff like the Shure wires which has to be another advantage. It's really hard to read which is left and which is right (but after you find out once you'll know forever based on their shape). In addition, it has a hole that equalizes the pressure between your ears and the outside, which is a good feature. Consistent with Sennheiser's other cans, this is extremely comfortable and easy to wear. I guess that deserves a 1/2 star boost.
Now for the negatives.
Design flaw #1: Isolation: I decided to take it outside near cars and busy activity to see how well it isolates. The first alarming thing I noticed was how loudly I heard the door close on my way out; not a good start. But I readjusted the tips and remained hopeful and optimistic . . . until I hit the street, when I was overcome with disappointment after hearing everything. I stopped, played with the tips, did every trick I could think of. Nothing. This isn't as bad as an open back headphone, but it's only a little better than a closed back. Sennheiser, don't you think that, perhaps, people wear IEMs to block out noise? If they want to hear outside noise they could easily wean an over the ear or on the ear can. Eventually, the only way I could listen to the music was to turn the volume up to a level that I later measured as 70-75 dbA (for comparison, the Shure SE846 isolate well enough to easily listen at 60-65 dbA). Thanks for the hearing loss Sennheiser!!! All the outside noise of an over the ear and the "pumping music directly into your ear canal to destroy your auditory hairs" dangers of an IEM. Great lose-lose combination you got there! Due to the risk of permanent hearing deterioration caused by this unacceptable and inexplicable design feature, it has to lose 1 1/2 points. (Edit: I measure dbA by putting the IEM tip right up against the port of a decibel meter).
Design flaw #2: Line noise. At first I decided to try it with the line hanging under my chin, the way you're supposed to wear it. But as soon as I started walking, it sounded like I was in a wind storm! There was no wind, but every time I would move, the chord would move, and apparently when the chord moves it has to let me know by sending this loud sound up and into my ear canal. Thud, thud, thud, thud. I get it chord, we're moving. You don't have to tell me every half-second. Let me move a bit without you machine gunning my ears. Sennheiser, did you even put this on and go for a walk with it before you decided to sell it to the public?! I wasn’t even exercising heavily. This is unacceptable, it has to lose 1 star over this. Edit: Sometimes I wonder if Sennheiser is playing a joke on us with this product. I say that because I just discovered that the Sennheiser HD800 has absolutely zero line noise. I tapped its line, shook it, whipped it . . . but I couldn't get it to produce even a tiny bit of line noise. So the HD800, which is designed for stationary use, has absolutely zero line noise, and the IE800, which is designed to be used while active, has insufferable line noise? Really Sennheiser? The fact that Sennheiser knows how to completely eliminate line noise means this flaw deserves another 1/2 star deduction.
Design flaw #3: So to cure the slight bit of chord noise you hear in every IEM, I discovered this trick I do whereby I let the IEM hang behind my head instead of under my chin. With the SE846 you hear nothing if you do this. I think your hair acts as some sort of sound baffle or cushion or something; whatever it is it works, and it helped a good amount with the IE800 too, but not as much as with the SE846 and the IE800 wasn’t as comfortable hanging behind your head. But whatever it's tolerable now and so I'm walking along and . . . I decide to put my player into my pocket, at which point I discover that the chord, from your ear to the jack, is only about 3 1/2 feet long! Sennheiser, did you test this on little people?! Did you think that perhaps customers would like just one measily foot of extra chord?!?! What terrible thing did you think would happen if you included a five foot chord like Shure? Fine, keep your chord short, but if I'm paying you $1,000, could you at least have thrown in an extra chord of a longer length? Shure includes two chords with the SE846 and their chords look to be of higher quality than yours, so why can't you? This inexcusable chintziness and lack of thought has to cost it 1/2 star.
Design flaw #4: This is minor but it shows how inconsiderate Sennheiser was when designing this. The case has no room to insert your extra tips. This four inch, by 3.5 inch, by one inch case (measure that out so you know how big it is) doesn't have room for the extra tips. The inside of the case is almost entirely foam, with a little groove where you insert the IE800 (you don't put the IE800's wire in the case, by the way, rather you must wrap the wire around the case). First of all, what happened to just being able to throw your IEM into a case and take it out with ease, rather than having to perform a complex lego piece insertion and removal each time? Back to the main point, Sennheier, if you're going to make a gigantic case as big as a fat wallet, could you let us use some of that room?! Cut a little area out of that foam where we can insert the extra tips you gave us. This is a small thing, but it shows such a lack of consideration and thought. It has to lose 1/4 star.
Conclusion: Ultimately, I am not going to keep these. Although I love the sound signature of the HD800 and this reminds me of that, I can't support a product engineered this poorly. It's of no use to me because if I'm in a loud area, I'll use my beautiful SE846 and if I'm in a quiet area I'll just use my HD800. This leaves the question of what rating to give these. Although the sound is very good, based on its hefty price tag it can't get more than 4 1/2 stars on sound (I have to save 5 stars for something that really wows you). Subtract 1 1/2 stars for lack of isolation, add 1/2 star on comfort, subtract 1 star for line noise, another 1/2 star for having line noise even though Sennheiser knows how to completely eliminate it, subtract 1/2 star for the chord, subtract 1/4 star for the case, and (this is very subjective) add 1/2 star for having something close to the HD800 sound signature (which I like) and for being made by Sennheiser, a very ethical headphone company that lets you return things you don't like for a full refund. So 4.5 -1.5 +.5 -1 -.5 -.5 -.25 +.5 = 1.75 stars, which I’ll round down to 1.5 stars because I expected better from a $1,000 headphone. For comparison, I gave the SE846 5 1/2 stars (capped at 5 stars). As anyone who has spoken to me on this board knows, I am a huge Sennheiser fan so I wanted to like this but this is just pointless.