I got one last month... just to check out and because it looked pretty darned cool. Compared to the Fenix, it is slightly more bulky though if you didn't have a Fenix, you would be hard pressed to call it bulky. The finish isn't as nice as the Fenix and it has a cheap plastic lens while the Fenix is AR coated glass. Brightness seems very similar... mine might be slightly brighter than my best L1P. M RR has a slight violet tint which is my least favorite tint but I doubt I'll ever use it very much. The clip is much stronger than previous River Rock clips.
At some point I'll try to open it up and mod it with an SSC LED.
After about about 8 or 10 hours of runtime on NiMH cells, I've got 2 straight runs of 30 minutes on a 14500 cell and probably another 20 minutes of various playing. My guess is on an NiMH cell, it's putting out ~25 lumens and on a 14500, it's about 35 to 40 lumens so there isn't the radicaly jump in output or shift in tint with LiIon that there is with the L1P v2.5. It gets warm, but not hot at all if you are holding the light. My guess is the LED can handle the LiIon just find but we can't know how the components will last on LiIon until someone gets MANY hours of runtime with it.
I don't have a light box or a recording meter, but sometime this week, I'll do a direct comparison of runtime against my L1P v2.5. I'd expect them to have similar runtimes.