Built a custom watercooled system for a new rig I put together a few years ago, ran great.
Then I decided to change the videocard... didn't feel like swapping the videocard waterblock and adding another to the loop(went from a single card to a crossfire setup). System still ran great just cooling the CPU. Then I had to swap the motherboard... complete pain in the ***. Ran great with the new board.
Decided to build a new system last year. With major component changes being a massive PITA with watercooling, I opted to go back to air. The cheap sealed units would be easier, but they simply aren't good enough. Putting together a good custom setup wasn't hard, maintenance was easy the couple of times I flushed it due to smart planning of tubing, but having to gut the entire thing for a mobo change, or needing to re-do tubing/blocks for a videocard change was just terrible.
I see that there's finally a waterblock available for my gtx590, but at $150, plus a CPU block, plus a pump, plus radiators... it's just not worth it.