I've had mine for a few weeks now, and I love it. The important factors for me are image quality, ease of use, and discretion.
The image quality is superb. The aps-c modules produce very high quality files that are at least equal to current aps-c dslrs. The images are sharp and clear, and have no significant vignetting. When processed with Lightroom 3 the raw files are super malleable and I can pull lot of detail out of the shadows, although I find that the Sigma compacts and Leica M9 are a bit better in that department. Likewise, I think that the Sigmas and Leica have a bit more room in the highlights as well. Therefore it is more important to expose properly with the GXR, whereas the Sigma and Leica files have a bit more room for recovery.
The GXR is very easy to use. The rubber grip and design makes it sit very nicely in the hand, although the Canon G10 is a bit more comfortable for me. All the buttons and dials are in the right places, and important things like EV comp are a button's push away. The 50mm and 28mm modules are a bit lens-heavy, though, moreso than the Sigma DP1 or DP2. The grip is therefore a necessity and not a luxury when using the aps-c modules. Once you've customized the buttons it is by far one of the best setups for one-hand shooting.
Startup time and noise got me a bit, though. The GXR fortunately remembers the last focus position when it was turned off, and goes back to that position when you turn it on. This makes it fast to focus if you are shooting things at a similar distance to the last time you used the camera. But the overall startup time is slower than, say, the Canon G10 or S90. Fortunately it is way faster to start and focus than the even the Sigma DP2. Shutdown time is likewise immediate and fast, unlike the Sigma cameras that can take their darn time if writing a raw file when shutting down. The DP1's lens will stay extended for a number of seconds until the raw file has been written. As a tangent, I wish a feature of all cameras would be to retract the lens FIRST when turning off, and let the file write even with the lens retracted.
The noise of the GXR surprised me a bit. The lens extension sound is a quite scratchy with the 28mm modules, and the shutter sound is quite sharp. It's not an issue in most circumstances, but last weekend I was shooting in a silent art gallery, and every fsssszzzzhhhit of the lens was quite audible. Same with the shutter. The 50mm modules is actually a bit more quiet than the 28mm, which is a both surprising and a little disappointing. One of the things I love about the GRD III is that the lens extension, focus and shutter sounds are very negligible, and I was hoping that the GXR would be the same way. In fact, one of the quietest cameras I have is the Sigma DP1, which focuses and shoot with the tiniest click. The DP1 is even quieter than the much-vaunted Olympus XA, which makes a metallic 'spang' noise when it shoots. But it is as slow as molasses, desaturates at ISO 400 and above, and has a slow, slow lens. The GXR is much faster, better built, more versatile and produces more consistent image quality throughout its ISO range.
But these are nitpicks from someone who wants the fastest and most silent camera he can get. I shoot with the GXR all the time and sometimes take it with me on weight-critical trips and leave the M9 at home. The GXR and two aps-c modules weigh about the same as the M9 and one solid lens like the Zeiss 21/2.8. And it has autofocus and (crappy) movie mode, to boot.