Sure it's not worth $20, but it's a fun & humorous little RPG that is one of the most polished games on XBLA.
I can't really comment on how much this uses the source material or if I missed a bunch of 'inside jokes', but the game definitely has charm. It did make me laugh out loud several times, but I am sure it would be even better if I had researched that source material better before playing it. So I basically went into this game as it was a new RPG with an original story & characters. With that mindset I enjoyed it very much. The game is obviously set-up for multiple episodes, and I am intrigued on where the story will go.
The big knock on this game is the fact that it normally costs $20 and it's very short. After paying only $10 for it, I feel I got my money's worth. If I had to pay the full price, I would have probably felt shortchanged. In reality, this should have been a $15 game especially for how short it is. You do get excellent graphics and top-notch presentation for an XBLA title. The music is great and I really had fun with the gameplay (except for a few annoyances with the battle system).
So in the end, is this worth purchasing? Well, if you are a fan of the source material and plan on checking out the other episodes.....well, then I would say yes. You will probably get more enjoyment out of the characters and just overall feel of the story than I did. If you are like me, a newcomer to 'Penny Arcade', then I would urge you to give the demo a shot. The other episodes look like they are going to be priced at $15, so $5 more for this game isn't really that much. At any rate, it certainly wouldn't hurt to give the trail version a shot and see if it wins you over like it did me.