Pros: Build quality, accurate mids, deep but well-behaved bass, detachable cables, excellent isolation
Cons: Sparkle could be better, not the most comfortable, don't love the memory cable
Bought those as a portable go-to set (subway use, airplanes, travelling, etc.) and am so far very pleased. I previously used Etymotic IEM's (ER-4P and HF3) which are known for their clarity and brightness but also lacking in warmth and bass impact. However, after loving the sound of my new over-ear full-size Sennheiser Momentum's wanted something a little more fun and dynamic sounding. The SE215's fit the bill. They have a nice warm sound with emphasized (but typically not overpowering) bass, very good and forward mids and slightly rolled-off highs.
The dynamic drivers sound fun and are engaging. The isolation on these is superb, easily blocking out subway noise without having to turn it up high (I use the black foam tips, also know as the 'olives').
On the negatives the sound could benefit form a little more sparkle and treble extension but otherwise the music coming out of these is very pleasing to me. On occasion (depending on genre) the bass can also sound a little over-emphasized if you're paying close attention. But more often than not it basically makes me rock my head! Basically if you like the full-size over-ear V-Moda M100's these will provide a similar sound and impact in IEM form but with better and more forward mids but less emphasized treble (less V shaped sound profile). Surely, there are better IEM's in the market (including higher-end models by Shure) but as the title says these provide great bang for the buck.
Other complaints are that I don't find them the most comfortable for long durations but I think that may be par for the course for IEM's as many people don't love Etymotics either (though I always thought they were quite comfortable with foam tips).
All of this has to be put in context however and for $99 these are just great sounding headphones with very good isolation capability and build quality. In fact, I think the same comments would apply even if they cost twice as much which to me makes these achieve a great price to performance ratio.