I tried the giro atmos and the Bell Sweep R. Both look sweet, are light, have lots of vents and do a pretty good job of minimizing the "mushroom head" look.

But ultimately you have to pick a helmet based on how it fits your heed. I have the Giro Atmos and love it. Sometimes I forget its there. Before I got it, I wore an old, cheap mountain bike helmet that was heavy, uncomfortable and no air flow. On a ride shortly after I got my Atmos I suddenly panicked because I thought I had forgotten my helmet. I actually had to reach up with my hand and touch it to believe I was wearing it. I know this sounds impossible but I swear its true.