Well known mostly-Nikon reviewer Thom Hogan has reviewed the Ricoh GXR, along with the P10 module, both the 28mm and 50mm macro A12 modules, and the VF-2 Electronic Viewfinder. I won't spoil it, but he says he's come to prefer the GXR to his Nikon D3100 for times that call for a lightweight kit.
I enjoy Thom's reviews, probably because his ideas about the ways cameras should work mostly line up with my own.
In the early days of digital photography, many people bemoaned the fact that you had to buy a new body to get a new sensor, Ricoh has solved that problem with the GXR. Although a lot of the complaints I've seen in various forums seem to see the this as a limiting factor. I don't. I see the body-sensor interface as the heart of the GXR system.
Already Ricoh is working on an APS-C M-mount module, and I can imagine other possibilities. How about a full-frame M-mount module? Or an APS-C Sony Nex module? And I'm sure we'll see other bodies for the GXR system eventually as well. Perhaps an improved version of the current body, hopefully with the hot shoe centered over the lens this time. Another idea is a body with a built-in EVF and swivel LCD, something like the Panasonic GH2. Or perhaps a body with a built-in OVF or Hybrid viewfinder similar to the Fuji X100? So, you could have your X100 style body for photowalks and traveling, your GH2 style body for more considered studio work or perhaps for video, an M module, a Nex module and a couple of native Ricoh lens modules. That sounds pretty good to me.
My recent purchase of a GRD2 has made a believer out of me about the Ricoh way, and I'm definitely keeping an eye on the GXR system.