i havent played any of the co-op mode yet, that alone tho could make it worth it, if its done right, and Valve usually does. Also, the reason Valve gave PS3 owners the PC bonus is because Valve had said any content updates they do will be free via steam, and MS said no all DLC will need to be paid for, and Sony just said ok fine by us! So that tells me Valve has plans of releasing new content as time goes on.
also if you get the game from Amazon, they did change it from #55 and a $20 credit, but now its $60 and a $15 credit. if you have a PS3 and PC, get the PS3 version and get the PC version free, so with the credit its like getting PC and PS3 copies for $45, and thats not a bad deal.
to each their own tho!
At most I'd pay $20 for a copy. The game is about 6 hours long on average with 0 replay value, no story, no combat. It's really, really good puzzle game, but there are far too many long, fleshed out games that give you a lot more to spend anymore than arcade price for the game.
Valve said there's about 2.5 hours more content than the first game, and there's a reason why the first game originally came packaged with the orange box, and that was cause it was extremely short and not anywhere near a retail price. 2.5 hours extra is still not worth more than #20 when you can just rent it and beat it and the co-op over a weekend.
As you said, though, everyone feels different so it's no big deal.