This is from a buddy who had one. I have no personal experience.
Seat is very uncomfortable, like less than an inch of padding. Not good for long stints. The riding position is a bit cramped in the legs with a reach to the bars. He said it was a perfect trackday toy, or any other time you don't spend much time on it, but it wasn't comfy to ride.
The torque was unbelievable. He was used to a progressive powerband and turbine like increase in torque as the RPMs climbed since he was a four-cylinder rider, and it surprised him a lot. Said the torque curve was so flat, the "sensation" of acceleration wasn't there, but before he knew it, he would be over 120 and he would "feel" like he was only going 60.
Said the fuel injection mapping was off and it didn't like low-RPM part-throttle. Kinda stumbly and jerky he said. Driveline lash too. Said it loved WOT, and you could tell it was "racebred" because it only liked to Run, not putter about. I've heard this from other RC-51 owners as well.
Can't remember much more than that. I wasn't in the market, but if I was gonna get a twin, I would get a Ducati. He agreed. Bear in mind this is opinion only, so...