Pros: Comfortable (incredibly so), small profile, excellent imaging, no microphonics, decent build
Cons: Mediocre isolation, best with after market tips
Note: the star ratings are not absolute, but instead relative to its street price at this time (~$55)
The pl50 represent a good value in budget IEMs, provided you like its particular sound signature and quirks.
The pl50s have a tiny form factor. They are easy to insert and remove, and are more than flat enough to lie down in. Needless to say, they are among the most comfortable IEMs available on the market. Build quality is also fairly good. The strain reliefs are well made, and they actually included one on the Y-split. They are durable enough to take to the gym, although I still wouldn't abuse them. There are a lot of accessories thrown in, but the stock tips aren't great. I recommend purchasing some sony hybrid tips (EPEX10A). Also, unlike the previous reviewer, I found this earphone to be virtually lacking in microphonics (cable noise).
The sound signature is mid-centric, and is one typical of balanced armature IEMs - namely, a lack in bass quantity. Bass quality is good, although the pl50 won't hit the low 20Hz notes. The mid range is this product's strong point. It is clear, articulate, and very forward. Treble is extended, but completely lacking in shimmer or sparkle. Whether this is a good thing or not will depend upon one's own taste. There is also a bit of warmth to the sound, and the soundstage isn't particularly great.
The pl50 is best with vocal centric music, as well as folk or country. At these genres, it excels, and by a fair margin. Especially for vocals, I doubt you will find a better IEM at this price point. On the other hand, bass heavy music is not this IEM's forte. Also, I will say that at times, the pl50 sounds a bit thick, so music that requires alot of air and space, such as classical, is not reproduced well. And like most balanced armature IEMs, the pl50's reproduction of timbre is not perfect.
Try to think of the pl50 as a very polite and inoffensive sounding earphone. I imagine that if you enjoy the sennheiser sound and don't mind and/or want less bass quantity, you will like the pl50. Their ability to stay out of the way makes them a great earphone for reading or studying. If, however, you are enamored with a more energetic presentation a la grado, you will likely find listening to the pl50 a rather boring experience.
So, in short, the pl50 is a well designed and built, supremely comfortable IEM. The sound is very good - for particular genres, and for certain tastes. But if you listen to lots of folk and vocal centric music, this is probably the overall best in ear monitor available at its price point.