Hi Shrap-
As far as utility is concerned, I think it's comfortable enough to use that way (between the fingers) only if you're used to doing so. I think it's a more natural motion to use the thumb-on-tailcap method when using the flashlight for regular activities.
Aside from that, the CombatLight grip was an idea bred from former FBI Special Agent Bill Rogers. The design intent was for the application of the flashlight-handgun technique which ultimately evolved into the Rogers/SureFire technique (with the CombatLight grip design) - other than what already existed, such as the Harries, Chapman, FBI Techniques and other qualified means of tactically applied light sources.
Anyway - besides the M2 and C2 as you mentioned, there's also the M3, M3T, Z2, Z3, and G2Z with the patented CombatLight grip.
And welcome to CPF! /ubbthreads/images/graemlins/grin.gif
-Jim