Of my most two used 2x123, lights, one is a Vital Gear with a modded G&P 3W drop-in. Diameter-wise, it fits in the head just fine. It is like .5mm too long and thus the head cannot screw down 100% to the, uh, in between adapter part. (Don't know what it's called.)
As mentioned, the fairly strong clip that allows for bezel down carry was a primary factor. As far as machining/fit/finish, I'd say it is 95% as nice as a Surefire... maybe even closer to about 98%. I'd call them interchangable quality. On the finish, I usually keep my nicer lights in a seperate pocket, but I don't baby them. I use them outdoors and nearly every one has had to have dirt cleaned from it on more than one occasion. My F2 looks perfect.
Suffice it to say this: I've had probably 30 or more 2x123 lights, bodies, heads etc and for me, my F2 with a U-bin modded G&P module is the best solution for me.