My only first hand is with Koolance and I'd probably get another one when the time comes. I got the PC2 case (turnkey kit), with a CPU and GPU block. Pros and cons:
Redundant pumps, though new models don't have this
Safety features - system will shut down if sensor detects threshold temp exceeded
Fan termal demand loading and three fan modes - it's quiet.
Excellent waterblocks, though finding reviews is nearly impossible
Great case design (modified Chieftec) and setup was a breeze
Radiator wasn't removable to clean (I fixed that by replacing rivets with screws)
New models are expensive
Worth considering I think.
Edit: From all I've read, Exos is a really good performer, but I don't like the idea of an external box hanging off of my PC. I'm constantly in it or moving it.