Having tried the app for a few hours, I will say this thing is filled to the brim with annoying bugs that render the app almost useless.
1. While playing music, if you get any sort of outside notification sounds (such as receiving email, or IM chime, or even keyboard clicks within the app itself!), Napster jumps the volume to the standard line-out volume. Once this occurs, you can't change the volume down with the buttons or within the app. You have to pause the song and unpause to access the volume controls again.
2. I've saved a few albums, but for some reason, after one or two songs, playback will just stop, often in the middle of a song.
3. Much of the Napster library is composed of "Buy-only" tracks. For those tracks, you can only listen to or download a 30-sec preview.
4. This is the big one: Sound quality sucks big time. Highs are easily distorted, and everything else is muddled. I'm guessing 64kbps bit-rate or something.
Promising, but feels like an alpha project at this point. I might use it for the next week (free trial), but I would definitely not pay $10/mo. until they come up with an even slightly better experience.