If you like the movie (/show), than you will like this. If you like other Joss Whedon work, than you need to check this show out (at the least).
It's all the music in the entire series (including intro and exit instrumental's). It's a little pricey as it's all the shows 14 songs, but half are shy of 2 min (quick songs). However, it was 100% independently written, performed and funded (mostly by Joss Whedon's family and friends). In those regards, you can consider this actually pretty well done and profits pretty much going to only them. There's not much more to say.
In the end, those who are fairly big fans, or something more, of Dr. Horrible's Sing-Along Blog will definitely want to have this.