You know SVTNate, I'm kind of in the same boat as you. Looking for a light for my USP .45.
I'm thinking of going for a Surefire light for two reasons:
1) Easy to detach and use as a normal light if need be...
2) Going with one of the new rechargeable ones to have the ability, well, to rejuice it.
I'm kind of wary about any light that drains the battery in such a relatively short amount of time, as practice with the light on and mounted would certainly be of paramount concern. If it went quick and I had to sport for one of those batteries, I might be distracted by $$ instead of practicality.
Either way, I'm sure any of these lights would be a great addition to the USP. I'm going to follow this thread closely for comments on any of those mentioned...
BTW, I'm not sure if the M3 will fit on USPs...