I'm not really sure. All I know is he was very highly rated at Spurs and then for some reason they decided to sell him last summer having never really given him a chance. It's a bit mysterious.
Personally I'd rather we went and bought a £20m+ defender, the absolute best we could afford, but I suppose at £8m or whatever Caulker seems to make sense too.
When I saw him at Spurs, he looked pretty good, though that was only for a few games that I saw. When I saw him at Cardiff, he wasn't impressive when we put 6 past them (well it's kinda hard to impress in that match when you let in so many), but I think he seemed pretty decent in another game I saw (can't remember which, but it was after us).
6m is a good price for a punt though. Even if he isn't a superstar, you should be getting at least a decent centreback for squad depth. Some other club would snap him up for that price though, e.g. Sunderland, who would need to replace the aging O'Shea and Wes Brown. They won't have Europe to offer, but Caulker is just 22 and has time on his side, and he'll be getting a lot of minutes there.
I wonder what candidates Carra might have in mind.