Well, at stock your CPU puts out 95w under load and maybe 20w at idle. To get that 20w to below zero you'll need something like 70-80w of TEC and a water loop that can effectively cool something putting out 90-100W of heat. No big deal.
However when loaded and putting out 95w the CPU will overload the 80-90W TEC and at best you'll have lousy temps, at worst things fry.
To keep the loaded CPU happy you need more like a 200w TEC. Now your cooling system has to be able to deal with 300W!
Worse from a 24/7 standpoint is that any time you're more than a few degrees under ambient temps you will start to get condensation on the cold bits. Water is a very bad thing to have on computer parts! Go subzero and you'll get ice too. Ice isn't conductive, but when you apply a load and go over 0c it turns into lots of water.
Better still once you overclock that 95W cpu and it turns into a 150W CPU you'll need 300W of TEC and cooling for 450W!
In my personal opinion sub-ambient cooling has no place in 24/7 computers due to this.
A system that effectively cools a CPU at full load will get it really cold at idle and cause condensation.
A system that effectively cools a CPU at idle will fry at full load.
It's an interesting exercise and I definitely recommend sub-ambient cooling for benching purposes (and by recommend I mean I'm horribly addicted to it), just not for 24/7 use.