I have the Corsair Hydrocool - earlier version - and it works great. It allowed me to overclock my 2.4 GHz Athlon to 3.2 GHz and it's showing 31.5 ºC right now. That is with a 77 ºF inside temperature (it's summer here in the desert, you know). When it isn't this warm, the temperature is usually in the high 20's.
Also the temperature trip points for auto kicking into "turbo" mode and for system shutdown are adjustable.
Only other point to make is that Tom's Hardware says that computer cases apparently miss the extra air circulation inside that you get with a regular CPU fan - in spite of not dumping the CPU's 80 watts or so into the case. They advise adding another fan to stir up air in the case no matter which water cooling system you get.
But I really have to recommend water cooling. Yeah, there is extra cost, but it isn't that terrible. And as far as the Hyrocool system goes, I was impressed with the overall quality though there was one issue I found when I opened the external unit up to look at the guts - there was a part of the internal tubing that had a kink in it because it was bent too tight. All I did was put some wire ties around that to force it back to round and all is good. I don't know if they eased off on that bend, or better - added an elbow, to the new units. Something to check though - regardless of manufacturer or model.
