Watercooling is the ideal way to cool parts that normally are problematic: HDD, GPU, SB, Mosfet.
But it's pretty expensive 'cause of the low demand. My cooling system is pretty old already. I upgrade some parts from time to time though. If you want to build your own silent highend system it's definately worth looking into.
So it's the same as a case basically. It can last several years and is easily upgradable/extendable.
Btw the CM HAF and Antec 1200 are designed for watercooling :P