I started wearing an HGU 55 last year for rollover and bird strike protection. After going on anticoagulants (warfarin/coumadin) following the installation of my new mechanical mitral valve, the helmet option became attractive due to the increased risk of head bumps producing brain bleeds with these meds (which I will be on for life).
My response to some of the concerns:
1) Canopy clearance: I am 6'3" and have the "almost an RV 14" mod in my 6A; I have no issues with clearance at all, and wouldn't with the seatback in its original position either. There is very little difference in height between my helmet and a headset with a padded band (some headsets I've used are actually higher), and the helmet is narrower than my head in any of my headsets. I have never hit the canopy in turbulence etc. Someone asked if anyone was aware of an accident where a helmet would have made a difference - I have a friend who flipped his newly completed 7A with his commercial pilot son in the right seat. Both are tall guys. My friend suffered a serious neck fracture which would not likely have been helped by a helmet (he recovered following a fusion). His son, however, suffered severe head lacerations from the shattering canopy, which I believe would have been greatly decreased or prevented entirely by a helmet. Just one data point.
2) Comfort and weight: I have a custom fitted Oregon Aero liner and Oregon Aero ear pads. The helmet thus equipped is FAR more comfortable than any headset I have ever used. Pressure is distributed over a much greater area, as is weight, which is actually very light. I basically forget I have the helmet on, which certainly does not happen when I wear a headset. Probably because of the high tech liner, I actually find the helmet very cool to wear; much better than my motorcycle and racing helmets in that regard. I don't fly in Texas, however, and avoid flying in the heat of the day when we do have our version of hot weather, so I'll defer to others on that issue.
3) Noise attenuation: I have a Helmets Inc ANR setup in my helmet; excellent sound quality and noise attenuation; superior to my old Lightspeed (cheapo) units and vastly superior to my DC H10.13.4's.
4) Additional neck stress: I am a physical therapist and regularly treat military pilots whose necks have been done in by high G combat maneuvering made worse by helmet design, or by years of rotary wing/nvg use. Very real stuff, but these are stresses that simply aren't reproduced on any regular basis in our aircraft. Sure, lots of people pull g's in RV's, but not the same g's, not as frequently, and not for the durations that fighter guys do. Furthermore, straight g loading while looking straight ahead is far more benign than what happens to your neck when you're looking way back over your shoulder in a turning fight while sustaining 6 g through 360 degrees of turn. Comparisons re: head contact with canopy or ground or acceleration forces comparing bare heads to helmeted heads are not particularly useful either, as most of us do use headsets which add weight and reduce clearance as well. The helmet I use is much, much lighter than a motorcycle or racing helmet, and would have far less effect re: acceleration injury to the neck than one of those would. My analysis only; your mileage may vary.
4) Looking like a wannabe fighter pilot: I am sensitive to this one, but my response to this is that anyone who thinks I wear a helmet to look cool has never seen me wearing it! Those who remember the Flintstones will understand when I say I look like The Great Gazoo wearing this unit. Not flattering. By the way; my son has an identical helmet which he got the old-fashioned way (issued while flying the T-38), which he is getting converted to use in GA aircraft (my GA aircraft, that is). Therefore, if anyone sees us in the 6A and accuses us of being jet jock wannabes, they will only be half right, as one is the real deal, so there.
In summary, I fully expected that there would be some tradeoffs when I decided to start using the helmet, but have been pleasantly surprised to discover there have been almost none; I prefer flying with the helmet in every significant respect compared to using the traditional headset and ball cap except that so far I am not using a visor and miss the peak on my cap shading my eyes. My .02. Okay, that might have been more like a nickel