I've had both G2 and Pelican M6; they both are bright, reliable lights. But, I think the SureFire is just in a higher class of quality and design. I gave the PM6 to a friend within a couple weeks of getting it because I found the general craftsmanship (fit and finish) and beam quality to be highly inferior to SureFire. Noticable artifacts and rings in the beam. The focusing ability was unnecessary for me; I've never wished for that feature in a SureFire.
For daily use, I also really like Nitrolon (the high-tech plastic that the G2 is made of), as it is "warmer" to the touch on a subzero winter day, lighter weight, won't scratch, and also won't scratch what it touches. Easier to hold by the teeth if needed. /ubbthreads/images/graemlins/smile.gif The G2 is more compactly sized as well.
However, the really comparable SF is the 6P; which costs $20 or more above the PM6. The 6P is gorgeous -- classic aestetic design and beautifully crafted compared to the PM6 -- but to be honest, for utility I think the G2 beats the 6P too. I've sold my 6P's, because while they're nice to look at and hold, I wasn't using them at all.
The only downside of the G2 is a lack of lockout tailcap. This hasn't been a problem for me, and you can unscrew the momentary G2 tailcap to achieve the same effect, or replace it with a G2Z tailcap, which would offset the cost savings of the G2.
And please don't think I'm raving because I'm a SureFire bigot; I just appreciate their efforts on the G2 a lot and I think it does represent exceptional value and performance. Again, many of its benefits are because of the Nitrolon construction, in my opinion.
Just my $.02, your mileage may vary.