Caveat: I was a playtester for the game. So, take this reply with a grain of salt.
It depends on what you're looking for. For me, the game is too much a departure from the core d20 (D&D) rules set. I wouldn't even have a playtest copy if another member of our group hadn't given it back to me. On the other hand, another mutual friend bought a copy at a con just because all our names are in it, even though his isn't. There are flavor points and setting information that are probably worth $20. If you want a book to mine for ideas about a Hyborian game, this used copy is probably worth it. If you're mainly looking for the Conan OGL rules, it's worth the $20 to get this copy used or to pick up a copy of the pocket edition with the corrections but not the fancy layout. If you want a corrected copy, get the Atlantean Edition or the pocket edition. If you want the rules and the nice layout, etc.; then get the Atlantean Edition.
I think I would find it especially worth it if you can trade in some books from your shelves that you're no longer using for $20 worth of credit to pick up this game for no money out of your pocket. Then you can read it and see for yourself essentially for free. One gamer's trash is another gamer's treasure, after all.
As an aside, I found The Road of Kings to be exceptional. I would expect anything by that author, Vincent Darlage, in this line to be good. Mongoose has called for freelance adventure writers for this game, so there may be some modules you can actually play one day.
