I use the Giro Xen, but that is a purely off road helmet (it has a peak) with more protection lower down at the back of the head, lots of pointing things to hit your head on off road, however I have tried over Giro helmets and I do find them very comfortable (as comfortable as any helmet can be on a hot day), it's all about what feels right for you, and one important point is that many of the more expensive ones do have better vents, it won't keep you cool, just not as hot as some helmets with not so good vents.
Personally I never wear one on the road, but that's my choice, and I definately don't want to be involved in any helmet debate.