Pros: Punchy Bass (especially for a pair of low-grade IEM's), Relatively comfortable, handle vocals very well
Cons: Incredibly microphonic cable, cable also very prone to tangling, tips tend to fall off
The easiest way to explain the Sennheiser CX-150's is that they do what you can reasonably expect. You can't really expect much from a pair of sub-50 dollar IEM's, and frankly, I was pleasantly surprised by the general quality of the CX150's. The sound quality is fine enough for portable listening from an iPod, as I use them, and I have yet to hear anything less than serviceable come from these IEM's. They're relatively comfortable, but the microphonic cable can get quite annoying at times. I tend to keep the cable under my jacket so as to avoid excess movement, but during summer, I can see this being an issue.
Comfort / Design
They're marketed as "twist-to-fit," and I still have yet to understand what that means (you can insert them just as readily as you can any other pair of IEM's). Comfort is a big issue of mine with IEM's, as I find it hard to find IEM's that stay in my ears readily. The CX150's don't disappoint, as I've found them very comfortable and stable in my ears. The biggest con here is the cable, the IEM's themselves are fine. The cable is clearly made out of relatively cheap plastic: very stretchy, microphonic, and prone to tangling. I've had no issues with the connectors to the jack or the drivers, though. The cable is fastened very securely to the respective ends.
Again, the CX150's do their job. Don't expect premium sound from these, as many would consider them near-disposable. That being said, I'm still very surprised at how generally good they sound.
The bass is really where the CX150's shine. I've not heard a cheap pair of IEM's such as this that do as good a job with the lower regions as these. The bass is punchy and powerful, but never really overwhelming. This may be personal preference, especially as I listen to a lot of Electronica (especially Electro House and other bass-heavy genres), but I never feel too put-upon by the lower regions, when using these phones.
The mids are nothing to really write home about; the mid-region is perfectly adequate (exceedingly average, as someone else put it). Vocals (male, especially) really do come out in clarity, though. Even with full instrumentation, one can still hear the vocals over it all.
The highs are really where you can tell that these are cheap phones. In Shostakovich's Sting Quartet No. 8, one of my favorite pieces, the upper register of the Violin is slightly distorted, and almost painful to listen to. It's supposed to sound very eerie and dissonant, yes, but on higher quality phones, you still gain a sense of warmth that is not found in the CX150's.
These are a perfectly serviceable pair of headphones if you listen to anything that doesn't have prominence in the upper register. Don't get me wrong, the highs are listenable, but only barely. I recommend these to anyone who listens to bass-heavy music and is looking for a pair of cheap headphones that'll do their portable MP3 players justice.