For me the main driving force for water cooling was getting the heat out of the case and the noise down.
I have two PCs with custom loops (CPU & GPU) and one PC with an AIO and that one is the noisiest because it's in a small (midi tower) case with 2 fans on the radiator, 2 case fans and the GPU fan running fast (because I run the GPU flat out doing BOINC stuff).
My main PC has a 200x200 Phobya radiator fitted externally with a 180 SilverStone Airpenetrator fan throwing the hot air directly upwards.
The fan runs around 800 rpm and I can't hear it normally.
The D5 pump/reservoir is mounted in the 5 1/4" drive bays and I can hardly hear that either.
To be fair it took about a month to get the microbubbles out of it before it became that quiet.
To compensate for the lack of cooling over the motherboard I fitted a 60mm fan onto the side of the northbridge heatsink where a little bleeds over to the VRM area too.