The finished product is fun, and you know your rig is always running cool. And it's not nearly as risky as everyone sees it. The liquid in both loops (TFC F1) is completely non-conductive. It will accumulate some conductivity overtime, but is pretty well an insulator until after 6 months.
The major, major downside is cost. You can ask Cabbs, Foothead, anyone. You simply can't get a real good performance loop for less than 400. By performance, I mean a quality block, pump, res, and rad, with good tubing, at least a liter of liquid, and fittings.
I'll go back and add up all the numbers to give you an idea of what all I've spent on liquid cooling.
Just shy of 900 dollars. :|