I feel that the $20 is well worth it. My wife and I played together, and spent about 3-3.5 hours playing the game. It was highly enjoyable. We laughed a lot, and enjoyed seeing one of our favorite comics come to life.
What else could we have done for $20? We could have seen a crappy movie in the theater (I don't know where you all live, but in San Francisco movies are about $10). This was much better than that experience is (save for something like Lord of the Rings, and I don't think we were expecting something like that from Telltale's first offering).
The best part is, this game is definitely going to have replay value. I will certainly be playing it again in a few months in anticipation of Vol 2. I expect after a month or two, it will still take me several hours to make it through the game. Also, I have already noticed that puzzles have a few different solutions - our virus scanner went off and bumped us out of the game, and we had to quit and go back away to where we had saved, so we played the Phoney and the possums sequence twice and solved the puzzle of acquiring the shovel a different way.
All in all well worth it - my only compaint would be that I was bummed that when I task-switched out of it, I couldn't get back in without forcing it to quit.
Thank you Telltale!!!!