Don't buy the RTR version, The kit is far superior.
I briefly had an RB E One R, or as we christened it at the track, the "Boner". I bought one on impulse from PBM when I was in there about a month or two ago. I built it and ran it a couple of times and the quality/performance was very good. The only reason I sold it was that someone I know wanted one and it just wasn't getting used due to the fact my main race car (Mugen MBX7R Eco) takes priority. The quality of the components is very good, and the parts all fit together very well.
The provided instructions are not the best due to the fact there are lots of hopup/extra pars included with the kit. This means that its not always clear which components to fit straight away although you can work it out pretty easily. The buggy handles/jumps very well although the box setup was a little on stiff side for the track conditions I was running it in, but the flip side of this is that it means its would be pretty good for bashing.
I have seen the RTR up close and the kit version is a lot better. The RTR has much more of an air of "cheapness" about it and is technically inferior, from the cheapened shocks (which incidentally mount on the front of the rear shock tower/arm on the RTR version) to the dogbone drive train etc etc, its just worse.