Disclaimer: My experience is only a single data point, I have not sampled nearly all IEM products and combinations, and realize everyone's preferences, tastes, physiology and expectations are relative/different. And without greater empirical data to establish my point as part of a potential trend, it really means diddly squat nothing :)
TL;DR...Bulletproof dynamic IEM with awesome "out-of-head" expansive sound that is an excellent jack of all trades. There is a reason why Sennheiser flagship models remain unchanged for lenghty periods of time and command a high premium. These might not be the latest and greatest in technology, bling bling or gimmicks (10 Balanced Armature drivers yo!!!), but they will still faithfully serve decades from now.
Full $0.02...I have owed these for about 2 years and they have been my main IEM during this period. I would not be surprised if I have 1,000+ hours of use on these (I travel for work 90% of the time). Below are my experiences;
(1) Main reason for favoring and coming back to these is their "out-of-head" open sound; I just don't "feel" like I am wearing IEMs when compared to same level Shure, ER, Westone, etc offerings. I have wore/listened to these through 12 hours work days without fatigue.
(2) These are also extremely easy to drive and do not require an amp to sound their best unlike many other flagship IEMs. Therefore, these will sound great straight out of a laptop or MP3 player. I have not tried them with an amp, but I assume it will need to be a really sensitive one.
(3) In typical Senn fashion, these have tank-like built. Their design is not fancy, but extremely durable. I typically store these in a Pelican 1010 case, but threw them in my briefcase or slept though the night wearing them without any issues. Same with he cable; I am a klutz and keep yanking these out. IMHO People that complain about Senn build quality are extremely ignorant/negligent and expect everything to be build of unobtanium.
(4) I like the typical Senn balanced/neutral yet engaging non-dry/analytical sound signature (versus, say, Etymonic Research) and ability to adjust the bass when I want to rock out (but most of the time is at 1). Outside the above quality, these are jack of all trades but master of none. Therefore, these may not fit people with very specific demands (bass, transparency, sound stage, clarity, etc).
(5) These are not deep insertion IEMs; therefore, their passive noise blocking is probably mid-pack (but still very good)...but this also what lends them to longer listening sessions. And same reason why they suck for workouts since they don't stay in when sweating (but they are never advertised to this purpose, so can't fault them).
(6) I paid $200 during Amazon's holiday lightning deal. Yes, these can occasionally be found for less on Woot or similar sites, but they always sell out in minutes (including the time I bought them and was lucky to sit on a fiber-optic network).
Amazon used to have "East Edition" versions, usually $20-30 cheaper than "West Edition" $250 everyday price, with only difference being user guide's language (not kidding)...but those seem to be discontinued now. Therefore, $215 is a solid price for anyone who does not care to spend hours chasing down additional $15-20 in savings.
(7) These would not be an upgrade to older IE8. I have A/B both and they just seemed to be stylistic refinement. And may be slightly better bass control at higher click levels. So if you can find those used, buy them. You will get same sound for 1/2 cost.