Colin,
I recently went through the same dilema.
Coming from using a Lomo LC-A, i saw the fixed lens of the GRD as being something very similar, and being able to control everything like an SLR moreso than other compact small sensor cameras it was very enticing.
The GRD1 gives an almost "film grain" type noise rather than a pixel compression. This is what makes this camera unique.
The GRD2 seems to use a more pixelated noise so a lot of original GRD1 purists dont like the new version and havent upgraded.
The GX100 caught my eye for having a few more features than the GRD1, features almost like the GRD2 but with a cheaper price tag.
Although i dont use a zoom normally for my shots with my LC-A, I saw it as maybe being a feature i might use. also being able to shoot 1:1 and faster RAW write-times.
So after asking a lot of peoples opinions - this isnt much help as most love either one or the other - and lots of pixelpeeping on flickr.com: looking at the quality of peoples pics, and weighing up all three cameras i decided to go for the GX100.
And i have to say im definitely NOT dissapointed.
Its fractionaly bigger than the GRD. It has an almost "inbetween" of the "noise" from the GRD and the GRD2.
A lot of people will go on about the lens quality of the GRD and the GX100 but to be honest, i've never been able to see the difference when pixelpeeping and im more than happy with the quality.
You can read as much as you like on the camera and people will rip into the fine details of these cameras pointing out each's pro's and con's. Its just down to what you'd prefer, what things you need in a camera.
It's not the camera that makes good photos - its the person who takes the shots.