I would have bought it via steam, but it was more expensive here in Australia versus other countries for some reason. So I bought my copy via play-asia.com for half the price it was being sold for on steam (AU$ 46 vs AU$ 89).
What I like about Steam, is that all your games are stored in there library, so if PC crashes you just down load it, and you don't need the game in the disc drive. Over Christmas they had mega sales, I know that some games were as cheap as £0.99 I bought quite a few one being Mafia 2 and other Flat out ultimate.