I bought this holster combo for my S&W M&P 9, 4.25".
The holster works fine for the gun. There is a little space around the gun (the holster seems like it was designed for the larger Glock 21) but it doesn't seem the space hampers the holsters functionality. This is my first Serpa/autolock holster, and 95% of the time it presents no problems, but occasionally I find myself struggling to press in the trigger guard lock to remove the gun. I suspect that a little more practice should take care of it. The "Level 3" part I have not had any problems with, and can't see any reason not to get a "Level 3" holster. The extra retention doesn't cause even a novice like me any additional draw time.
Comfort wise, sitting cross legged can cause the holster to dig into the lower leg. Walking / running, no issues, although it can 'flop' around a bit (guess I need to tighten the leg straps?) The holster does stick out appreciably from your leg, so I have had to learn caution walking close to things on the holster's side.
On to the light:
Unfortunately, the light just doesn't work reliably on my M&P. The trigger guard on the M&P interferes with the trigger on the light. Support side activations are perfectly fine. I have a support case open with Blackhawk over this issue.
The light is plenty bright, light, and small. I estimate the hotspot about 4' at 20' distance. Spill is useable. Battery life is mediocre, but thats to be expected on a single CR123.