Summary: Just a few notes to qualify some of the ratings that would not be obvious: The highs are so strong (more than HD800 I think) that I can only give the IE800 a six. Soundstage seems OK to good for an IEM, but not unusual. Portability is good in the sense of size, but the carrycase design is fiddly. Sennheiser's isolation spec doesn't match my simple test with a music speaker playing from a few feet away.
Summary: The IE800 are the best IEMs I have ever tried. This includes the Shure SE846 & 535, Audeo PFE232, AKG 3003, JH13 and JH16 and many others. I also prefer their sound to many full size cans like the beyerdynamic DT-880, T70, T90; Sennheiser Momentum, HD650; Shure SRH1840 etc. The closest I've heard to the amazingly balanced sound of the IE800 is the LCD-2 rev 2, which I also own.
Summary: I'm not that much of an IEM guy. That said, I needed some IEMs as I sometimes travel. I wanted a good all-rounder, something with which I could listen to music on my phone and also play games on PS Vita.
I didn't want some "decent" IEMs though. I had those (Sennheiser CX-300 II, Audio-Technica ATH-CKX9IS) anyway. I wanted the best IEM out there. I've tried two that are said to be the best, Shure SE846 and Sennheiser IE800.
Pros: Amazing all around sound, superb bass, mids and treble, cool and modern design
Cons: Nothing for me. Non-detachable cable might bother some.
Summary: 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.
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
Summary: After reading glowing reviews of both of these in ear monitors I was still unable to decide which one would be right for me so I bought both, first the Shure and a few weeks later the Sennheiser. I already own the JH Audio 16 Pro, Ultimate Ears Triple.fi 10 Pro, HiFi Man HE-500, Sennheiser HD 650, Shure SRH940 among others. My music files are all in FLAC format and I play them through an Astell & Kern AK120 and also used a Samsung Galaxy S4 for comparison purposes.
Pros: Amazing soundstage, incredible detail, deep impactful bass, very comfortable even for long periods, easy to drive, sound good even at low volume!
Summary: I have the Sennheiser IE80 but wanted to try the IE800 to see if they were really all that much better. As it turns out they are. The IE800 has a smooth balanced sound signature from the base to the highs. The sound is 'clearer' than the IE80's as well. I love the fit, too. They are very small earphones, and can be worn all day without any concern for fatigue or discomfort.
Pros: Smooth and balance sound; excellent separation; small and comfortable
Cons: Cord a little short; cord microphonics noticable when moving around
Summary: Love them. Read a number of reviews and it was between these and the Shure se846. The Sennheiser's went on sale and I jumped right on them. The only really complaints I read about was about cable length and microphonic noise. To me cable length is fine I use this in a completely listening mode I.e. sitting in a chair and/or on a airplane or even in bed. No Microphonic issues.
Pros: Size Kevlar cable Sound quality
Cons: Noise isolation on plane ... again very minor issue. Didn't like case that came with it ... too big. I'm using a smaller case that came with my older Westone's that way I can carry in my pocket.