I'm only part of the way through the campaign right now, but it's the most fun I've had in quite a while. I'm surprised at the low reviews, as I would definitely give it an "A". I didn't like Bioshock and I never played Bioshock 2, and despite the similarities, I think this is a much better game. It also has some elements that remind me a lot of Half-Life 2. If you like HL2, then you'll probably like this. Either way, it's definitely worth a low price. I picked it up for about $15, and it's well worth it.
tl;dr - If it's cheap, buy it!
100%: Retail (18), Arcade (8)
Proudly haven't played Avatar: TBE