I had a student at an autocross with a brand new (only 2500 miles on it) RX-8 this weekend, and I was able to fit in it comfortably even with my helmet on, but the RX-8 in question was a Base 6-speed model with no sunroof. I'm 5'11", with short legs and a long torso, and headroom - especially with a helmet - is usually an issue for me in sports cars. However, the non-sunroof RX-8 had more headroom than my Miata does, plus it also beat out my old '98 Prelude and '93 RX-7 for helmet clearance.
Even better, I got a chance to drive the car at about 8/10ths on a 1.5 mile autocross course while showing my student the line. What a fun car! As much as I hate the marketing tagline, it really is the closest thing to a "four-door sports car" that I've driven lately. The ergonomics of the cockpit say performance car, not "sports sedan", with a lower and more reclined seating position, proper pedal placement for heel & toe shifting, supportive seats with excellent bolstering (this particular car had the cloth seats and I liked them), plus a steering wheel and shifter that fall readily to hand.
As a driving car, it was wonderfully communicative. The steering was very well weighted, offering excellent tactile feedback and a wonderful ratio for enthusiastic driving, the brake pedal had a very linear feel to it, and throttle response was very crisp. The car's turn in response was excellent - the RX-8 was as eager as a young puppy to change direction when asked, without seeming twitchy or darty at higher speeds. Must be that low polar moment of inertia, because I can't remember ever driving another car that offered the same blend of high-speed directional stability and lightning fast turn-in response. Transitional behavior was excellent - the RX-8 exhibited great balance, and was relatively easy to rotate with a sharp throttle lift or little trail braking, without ever wanting to come around on you.
As for the engine... I guess there's a reason I've owned two RX-7s. Nothing in the world is as smooth as a rotary, and the RX-8 is as smooth as they come. Throttle response was very crisp, and the powerband proved to be very linear - there are no peaks or valleys or cam switchovers or turbos spooling. Just a seamless, constantly rising amount of power. I found it to be a very tractable engine, and while it doesn't offer stump-pulling torque, the engine's desire and ability to rev freely made it easy to keep this car on the boil.
Overall, I loved it. I was immediately comfortable driving this car hard, much more so than in anything else I can recall having driven lately. It doesn't offer the same kind of brutal punch that an STi or Evo offers at a similar price point, but the RX-8 as a whole is truly greater than the sum of its parts. An excellent driver's car, and one that also appears to offer at least some amount of practicality.