Just got the Really Right Stuff BRX1 bottom plate with grip. I haven't had a chance to shoot with it yet.
First impressions:
It really gives the RX1 a secure grip, I feel that I'm much less likely to drop my expensive camera. The wide groove near the top perfectly fits my middle finger and my thumb naturally rests on the rubber pad below the exposure comp dial.
The grip is very well made of anodized aluminum and mounts securely with a quarter inch hex screw to the bottom of the RX1. There is a cut-out which allows the battery/SD card compartment to be opened without removing the grip. There's another cutout in the bottom for the speaker.
The bottom of the grip is a dovetail compatible with RRS and Arca-Swiss quick release clamps.
The RRS grip does add a bit of bulk to the RX1, and it's not cheap, but the feeling of security when shooting handheld more than compensates.