I was very intrigued, of course, by the DP1. But I've read quite a few reviews now and the general consensus is that its shortcomings don't quite yet make up for it's fantastic sensor and lens. This is the personal conclusion of the reviewers, of course - your mileage may vary.
For me, the dealbreakers on the DP1 are 1) saturation problems at ISO 800, 2) no macro mode, 3) poor LCD screen. Unfortunately, #1 seems to be an issue with the Foveon sensor and is not therefore likely to be remedied by the DP2. But who knows, perhaps Sigma and Foveon will figure it out.
The macro mode may not seem like an issue if you don't shoot macro (I don't either), but with a 28mm focal length it is necessary to get very close to your subject if you want to fill the frame. The minimum focusing distance on the DP1 is 11.5 inches, and that's not close enough. The Ricoh's focus as close as 2 inches, which allows for close-ups and also dramatic perspective shots with foreground and background objects.
The GX100 is indeed more versatile than the GRD1/2. However, it is also slightly larger, the performance of the lens is inferior (more barrel distortion, not quite as sharp) and the quality of the RAW files is arguably not as good. The GRD 2 writes RAW files faster and has some nice features (like a level) that the GX100 doesn't.
The GRD 1 has much sharper JPEGs than the GRD 2, as has been pointed out, due to NR that can't be turned off in the GRD 2. One would hope this might be addressed in a future firmware update. The GRD 1 takes FOREVER to write RAW files.
Also, the GRD 2 can shoot RAW in 4:3, 3:2 and 1:1.
I am still very torn. 28mm is not a familiar focal length for me, whereas I love 35mm. I can imagine getting the GX100 and using it with an OVF at 35mm, but then I still have the option to use other focal lengths if necessary. OTOH, I love the even smaller size of the GRD, and the sharper lens with less distortion (along with the faster RAW write speeds and the level on the GRD 2) are very appealing.
I guess I can always get one and if I don't love it try the other.