They run about -40C head temps and cpu temps run -15 to -25C (correct me if Im wrong, havent owned one yet). Basically, heat is not much of an issue with a prometeia vapochill or DIY phase change unit.
Great for overclocking if you plan on using it as a workstation (like me). Or want to brag to all your buddys. Either way its nice. Though its costly, about $500 for MachI $1000 for MachII.
If you think about it though, $500 isnt bad considering the performance gains and the fact that a good WCing setup can cost $250. If you have WC'ing already, like most who make the jump to phase, use it for graphics and chipset, then youre really rockin.