First off, let me start by saying that I didn't want to pick these up at first, even after hearing all sorts of good things about them on this site. However, as usual, I gave in and bought a pair. Given that TDK is relatively new to the headphone world, what they've accomplished here is pretty staggering.
Design/build: A laughing stock, to be frank. Not only does it look like a cheapo Chinese IEM, it has a pretty poor build. Visible seam running through the housing, a cheap feeling cable, wimpy 3.5mm jack. Although it is compact, it again feels cheap and shoddily-made. Not a good start.
Accessories: Here it only gets worse. Despite having an MSRP of around $200 (though I got them for half of that), TDK only gives you a cheap pouch, a shirt clip, 3 sizes of silicon tips, and 2 types of foamies.
Isolation: The IE800 is a vented IEM, and as such the isolation is only average. Relatively shallow insertion depth makes it worse.
Microphonics: The microphonics are horrid, accentuated by the fact that the flat cable makes it harder to wear over the ear. Shirt clip helps, but doesn't do enough.
Comfort: Mediocre. I think the problem here was that it was supposed to be a deep-insertion type IEM, but since the housings are actually quite thick, deep-insertion is impossible. As a result, it doesn't feel really secure when wearing it, and it has a tendency to slip out of my ears.
Treble: The IE800 has a very energetic treble, with lots of sparkle, almost up to the point of being bright. Clarity and detail-retrieval, however, are superb. At no point do these sound metallic or harsh.
Mids: In a sizable contrast to the treble, the mids are smooth and vocals sound magical. The midrange of these are prominent, a bit forward. Weirdly however, these IEMs are tuned in a way where neither treble or midrange feel detached. It just sounds right.
Bass: Despite that this is a dual-driver IEM, the IE800 are relatively light in the bass department, though what is there is fantastically tight and well-controlled. It doesn't extend too far, but it still sounds brilliant on bass-heavy tracks such as Scream by Avenged Sevenfold.
Soundstage: The soundstage is truly amazing for an IEM, and it even surpasses a few full-size headphones in my opinion. The imaging on these are excellent as well, giving these a immersive 3D quality.
Separation/detail: In terms of separation and detail, these are superb, even matching my HF5's in this aspect. Strangely, these seem to be far more forgiving than the Etymotics when it comes to low-bitrate recordings.
Timbre: These seem to be made for electric guitars, though with other instruments they still sound incredibly realistic. The decay on the TDK's are also fantastic, never too slow or abrupt.
Value/Conclusion: For the sound alone, I would eagerly pay the MSRP, and more. Compared with those in its price range, the IE800 are truly in a different league. It is for this reason which I can't criticize them for it's inherent shortcomings, of which there are many. Although something like a Shure SE535 or Westone 3 is a much better all rounder, if you're looking for sound alone, then save yourself a lot of money and get these instead. (Keep in mind this is only my opinion, feel free to disagree.)