We have two 2.5s in the office and they are no louder than the 1.8 (previous-generation) we have. The fans do rev up under load but apart from that, they are FAR quieter than the dual-G4s they replaced. Mine sits about 18 inches from my right ear and i barely notice it, unlike the constant roar of the G4 (which probably deafened me).
It's a tradeoff; you can have fans blowing loudly all the time, or have a quiet machine that occasionally kicks into high gear. Most of what we do doesn't really stress the processors, and when the fans do kick up they very quickly throttle back down again. Just don't expect the machine to be silent; it's not.
I don't think there have been any reports of trouble with the liquid cooling system, but if you're really concerned, or if you need to make your living with the machine, I recommend AppleCare.