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Borderlands is on sale for $4.99 / £2.49 at GamersGate and GamersGate UK as the Daily Deal. Major thing to note is that this version is the regular edition and not the GOTY. Even more importantly is this version won’t work with any DLCs. It’s possible in the future the DLCs will release on GamersGate which would work with this version but don’t count on that.
For now I’d say only pick this up if you just want to play the game now and you can’t wait for a better deal.
IMHO this game is all about co-op. Single player would be a little too boring. This game has an 81 metascore.