Well, I had the opportunity to go to my LBS and spend about an hour or so trying on every Bell, Giro, and Specialized helmet that is currently available. Unfortunately, those are the only brands they carry, however they have a huge selection. I'm fortunate that my LBS carries such a large inventory to try on.
I tried just about every helmet they had and the helmet that had the ABSOLUTE LOWEST PROFILE of all the brands mentioned above was the GIRO ATMOS. One helmet that comes in a close second place was the Bell Sweep (R or XC) and the Giro Pneumo. As far as MTB specific helmets go, the Giro Xen was the lowest profile when compared to any other Giro or other brand MTB helmet, but it didn't really come close to the Atmos or Pneumo as far as 'low profile-ness' goes, but I guess that's were the $50 - $60 difference in price plays a roll.
If you have the opprtunity to do a side by side by side (etc.) comparison of all these helmets, I think anyone would agree with my findings. Interestingly, each of the helmets fit me comfortably and, with the proper adjustments, they all seemed to fit as good as the next. The differences were in weight, amount of coverage in the back and along the sides, number of vents, width of straps, and general overall appearance. I couldn't really see how one would fit certain types of heads better than others b/c the Roc Loc system (as well as Bell's GPS and Specialized's PRO Fit adjustment systems) made it so that I can get each one to fit me perfectly.
Now, going back to the Atmos... As I mentioned it had the lowest profile. Additionally, it seemed to be the most comfortable because it was the lightest and had the most number of vents of any helmet I tried. Because I live in South Florida, ventillation is very important to me and the Atmos was more than adequate in that area. The Bell Sweep XC and the Giro Pneumo came in a close second (allthough not exactly as low profile) and had the added feature of a visor. The Xen was 'cooler' looking 'on the shelf' and appeared to offer greater protection, but the way I see it, any helmet made by any of these manufacturers were going to protect me and the differences in level of protection is negligeble. At this point, I'm shopping for looks and comfort.
That being said, I've gone ahead and ordered the Atmos in black/pewter and home to have it very soon. In the meantime, I've put my old Giro and Bell helmets on Ebay for just a few bucks and hope to defray some of the costs of my new $150 Atmos.
