Pros: great design, comfortable for long periods, natural sound, awesome replaceable cable
Cons: poor isolation, tunable bass port is almost useless, most accessories aren't useful
Let me start off addressing the pros I've listed.
--I love the design of the Sennheiser IE8s. They look great, it fits the shape of my ear, the cable goes over ear to reduce microphonics, and they are easy to put in.
--These are comfortable IEMs, I often wore them for several hours at a time to deal with noisy floor mates in my door.
--Natural sound is something I'll cover when I hit the sound section later.
--The cables on the IE8s are probably the best I've ever personally experienced. It is very well built, feels nice around my ear, supple, and most of all user replaceable. A bit disappointed that the plug isn't gold plated, but it's not a huge issue considering how great the actual cable is.
Now for the cons.
--The IE8s are infamous for their low isolation. If you are on noisy streets, buses, subways, etc., these may not be a great choice because you'll hear more of what's going on around you than you really need to without custom tips.
--The adjustable bass port was one of the big features I remember being talked about in the early days of the IE8s. I quickly found out that it's useless unless you like an exaggerated midbass and low subbass. As you turn it up, it only increases the bass response below the hump. Essentially it smooths out the bass and makes it more even. I just turned it up to max and left it. I find that most people use it on the lowest or lower-medium settings, and I strongly recommend at least trying it for a few days at max before making a decision.
--Most of the accessories that come with these IEMs are not useful. The tips are ok, I personally prefer UE single flange tips before I got custom tips for these. The small case it came with is a bit difficult and troublesome to use. The earhooks are too bulky and uncomfortable.
Moving to the sound...
The thing most discussed about these IEMs is the bass, and for good reason. I find that the IE8s produce bass really well. It is full, present, and textured. It has quality and quantity. Admittedly it can get a bit boomy on some super-bass tracks, but it holds well for most music I've thrown at it from all sorts of genres. When the bass knob is turned all the way up, the bass sounds smooth and even to my ears, with a very slight midbass hump. This hump is more noticable when the bass knob is turned lower. When amped, the bass tightens up a bit and there is more punch to it.
I think the IE8s have a very lush, warm midrange. It's very smooth and inviting. The most important thing is that it sounds natural. I've been through a few sets of headphones where male or female vocals just don't sound right but these produce voices spot on. I hesitate to say more about the mids because I haven't heard other headphones/IEMs which reportedly have awesome midranges, such as the Shure SE530s.
As for treble, when I first listened I thought it was a bit recessed. As I listened more though, I realized that the treble and mids were well balanced and that it was only an emphasis in bass that made that first impression. The IE8s have the greatest treble extension I've heard from IEMs. The treble is not sparkly and bright, so if that's the kind of sound you like these are not for you.
These IEMs have detail, despite how it may seem on the first few listens. Particularly, you might find that the treble lacks detail. I think this is simply because the treble is not emphasized, so it's harder to notice.
The presentation is definitely relaxed and slightly dark, which may again explain why they seem to lack a bit in detail. It's a presentation that doesn't suit everyone.
I sort of mentioned this a little earlier, but these IEMs sound very natural. They never sound strained and always seem to handle music with ease and finesse across the whole frequency spectrum.
I've owned these IEMs for almost 8 months now, the longest I've kept such a pair. I don't see myself selling them soon either, partially because I invested in custom tips, but also because the sound suits my preferences so well. I love the bass, I love the presentation, I love how effortless the sound, and I love how natural they sound.
Will you feel the same about them? I don't know.
I hope this review will help those who are considering the Sennheiser IE8s make a decision, but remember these are my opinions, my ears, etc., so take it with a grain of salt.