For a twin it's surprisingly revvy, with a fairly narrow powerband all things considered. It's unlike most twins in that the real power starts late in the revs, around 7k, and tails off immediately upon 9.5k, where the "soft" rev limiter kicks in (actual redline is 10k). Now, when I say "real power" I mean the kind of useful racing powerband that an experienced road racer will know how to use in anger; the RC51 makes street-usable power from very low in the rev range, like around 3k or so, which is only hampered by a ridiculously tall first gear. It's plenty powerful, but not nearly as powerful as the GSX-R 1000. Most people ditch the soft rev limiter to let it go to its full redline, that will get you a little breathing room, and there are all kinds of other mods that can get you power here and there. I've only put a set of Hindle cans and a PCIIIr on it and it's plenty fast for me. I've had it up to Summit Point, W.V. for a couple of track days; the first time up I saw 156 mph at the end of the front straight and was doing 1:22s (with no passing allowed in the infield) on Avon Azzarro AV49/50 street tires. It rained all day the second time.
Riding position is extreme (a little more compact than the GSX-R I believe) and the seat is like a vinyl-covered 2x4. I commute on it when the weather is nice, but it's really not that pleasant. I'm a shameless poser and the bike just looks so damn cool I can't help it. It puts off a lot of heat once you get into traffic (side-mounted radiators) and the tall gearing makes stop-and-go a pain in the ass.
Now for the positives; one, this bike has character all over. The overall feel is of something carved from metal. Some people say it's the least attractive of the sporting twins but I disagree; it reminds me of the last NSR's in it's styling and I've lusted after it since they first came out. The sound, especially with an aftermarket exhaust is unreal. You get that v-twin rumble on top of the really cool cam noise; in-lines sound pretty weak next to it, IMO. I find it handles very nicely and neutrally, but I've also found that tire selection makes a huge difference to the handling. I've run the stock D208s, the Avon AV49/50, and now Pilot H2s, and I like the Avons the best on it, the D208's the least. Personal preference and riding style will obviously come into play here. Overall build quality is excellent, aside from the multitude of strange fasteners Honda has used to secure the fairing pieces.
I love the RC51, but I do think it might not be everyone's cup of tea. And I must reiterate, it's not at all as bad assed as the GSX-R, at least not power-wise.