Although I am going to write this from the perspective of a GF1 owner thinking about the GX1, the points I will make are relevant to anyone thinking about this camera. The GX1 is, at last, a worthy successor to the legendary Panasonic GF1 micro four thirds camera. While being reassuringly familiar to GF1 owners - it looks very similar and feels very solidly built, it offers many upgrades. To start with, there is a new grip on the front of the camera which makes it much easier (and less risky) to hold it one-handed, and it just sits in the hand much better now. The control layout has seen relatively minor tweaks - the key thing is that there are still plenty of controls enabling you to access what you want quickly and efficiently. The main upgrades from the GF1 are fourfold: (1) the 16MP sensor in this camera has far better high ISO performance and lower shadow noise than the 12MP sensor in the GF1, G1,G2, EP-2, etc. At base ISO the GF1 and old 12MP are generally superb, so if you only tend to shoot at low ISO, perhaps using a tripod, this first gain is perhaps not that dramatic for you, but if you shoot low light shots you are going to see much better performance (still not APS-C performance though - the Sony 16MP APS-C sensor is still king for getting the balance between MP and noise just right); I am seeing about one to two stops better performance at high ISO than the old 12MP sensor; (2) the autofocus - it is MUCH faster than the GF1 autofocus, offers a new mode (AF-F) which is quite useful, and the touch screen offers the interesting option to touch for focus and shutter release; note, however, that continous autofocus tracking of moving subjects such as children playing sports, still lags behind the Nikon V system or DSLRs with Phase detect autofocus - but that is true for most mirrorless systems apart from Nikon's; (3) the JPEG engine is much improved, though as is usually the case, to get the very highest umage quality you need to process from raw - not using the included Silkypix software (good for colours but not for resolution) - I suggest you use Photoshop Elements, Adobe Lightroom or Capture One). In particular, high ISO Jpegs strike quite a nice balance between retention of detail and noise reduction. Sure, I can do better using raw and post-processing techniques and Photoshop plugins such as Neat Image or my current favourite, Topaz DeNoise, but this takes work and time; (4) the optional electronic viewfinder, the LVF-2, though expensive (around £200) is superb - it is essentially the standard electronic viewfinder present in the Panasonic G1, G2, G3 etc, and with its 1.4 million resolution is far better than the tiny LVF-1 electronic viewfinder for the GF1 (note that the LVF-1 will not work on the GX1 and that the LVF-2 will not work on the GF1 camera). The LVF-2 has a tilt function just like the LVF-1 and with the magnification of the viewfinder you get a big, bright image to look at. Other small improvements - the menu system is more extensive and more intuitive, there is now a digital level gauge to help you get straight horizons (though unlike in Olympus cameras, there is no calibration routine for it, which seems a shame); Colour is generally more pleaseing than earlier incarnations of Panasonic micro four thirds cameras - pretty much gone are the magenta sky issues and weird colour casts with landscapes that sometimes dogged my GF1 shots. Skin tones seem pretty good, especially when shooting raw. Dynamic range - well, my own findings don't fully agree with DXO Mark - DXO labs don't report much difference in DR between this sensor and the 12MP sensor used in the GF1 but my shooting has shown that the camera's exposure metering plus the sensor's DR seem to mean I get fewer blown highlights.
What about the touch screen - I don't list it as an improvement because I'm not sure it is. It is a bit better in direct sunlight, compared to the old screen, but still low resolution compared to what is on offer on most DSLRs, the EP-3 and the new OM-D. Panasonic is only now starting to introduce a 920k dot LCD panel on its micro four thirds cameras and I guess we will see that kind of panel on the GX2, but for now we are stuck with a 420k panel. My major problem with it is that when you magnify the playback view to check focus is achieved, it's sometimes hard to tell. It's actually more accurate, due to the greater resolution, to check focus using the LVF-2 add-on viewfinder. Somehow I found the screen on the GF1 a bit easier in this respect, even though it is the same resolution - I think these touch screens that we see here on the GX1 just seem a bit less sharp and contrasty. One big advantage of the touch screen is the ability to touch the screen for focus and shutter release, though to be honest any serious use of this technology will probably require a tripod to steady the camera. And once you are into the realm of tripod mounted macro work, which is where this touch screen feature is really beneficial, you would, to be honest, most likely be better off with a swivel LCD such as that offered by the Panasonic G3.
So, in summary: Fast autofocus (but not for moving subjects), strong build, nice new grip, lots of manual control, good 16MP sensor with good high ISO performance and good DR. Better colours and JPEGS than previous Panasonic models. The only real downside is the low resolution LCD and the high cost of the LVF-2.
What about the Panasonic G3 - isn't that better value and virtually the same or better than the GX1? Well I own a G3 and it is a great camera. It is only a bit bigger than a GX1, offers a built in EVF, the same sensor, same touch screen and almost the same focus speed. However, the GX1 has better build quality and is smaller if you detach the LVF-2 viewfinder. The G3 does have a tilt and swivel LCD, which is damn useful for macro. Overall I like using the GX1 more, because I am happy to carry the LVF-2 viewfinder in one pocket and the GX1 body in another pocket - some moan about detaching, re-attaching the LVF-2 but at least it doesn't keep sliding off the camera like Olmpus' VF-2 does off my EP-2 !
I heartily recommend the GX1 to GF1 users thinking about an upgrade - try one out a local camera store if you can. To be fair, you should also look at the Olympus EP-3 and the new Olympus OM-D, though these excellent offerings from Olympus are more expensive. If you are a macro shooter and don't mind more plasticky build quality, also check out the Panasonic G3 as it is good value for money and offers most of the features of the GX1 in a more DSLR shaped body.