Very much worth it for $20 in my opinion. It's a fun and quirky game. Like a few others have mentioned, it starts slow in terms of combat because you really don't start with any combo's other than X + X + X.
The "comedy" is very much crude though a lot.. some of it I didn't like, but I still found the majority of the scenes to be silly. Plus, it has High School of the Dead costumes!!
Going through the game took me about... 6-7? hours, but you can continue to play for ranking modes, better high scores, etc.