I had a Canon G7 that died just before a trip two years ago. Luckily for me, we were in a large city with a decent camera store (Penn Camera, now sadly deceased), so I had a range of options for a new camera the same day. I originally thought I would just buy the G10 or whatever the top Canon compact was at the time, but once I was able to handle the G10, the original Sony NEX camera, and the Panasonic GF1 side by side, the GF1 was the easy choice. With the 20mm f/1.7 lens, it has been my primary hiking and travel camera ever since.
That fall, we purchased my wife a Panasonic G1 kit with the 14-45 and 45-200 zoom lenses. I have since added the Panasonic 14/2.5 and the Olympus 45/1.8, and bought a Panasonic GH-2 kit when it was on sale from Amazon before the holidays. So we are pretty heavily invested in the m4/3 system. (Up next I want the 7-14 zoom and maybe the 100-300.)
I've been happy with the image quality and user interface of all three cameras, and the lenses are simply very, very good. The older cameras (GF1 and G1) are good to mid-range ISOs, while the GH2 provides surprisingly good images up to ISO 3200, and I have one shot of my daughter shot at ISO 6400 that makes an excellent 8x12 inch print. The fast prime lenses make a difference, too, in that I don't often need the nose-bleed ISO values to shoot usable photos.
My other cameras are a Canon 5D Mark II at home and a pair of Canon 1D Mark IV bodies at work (sadly, those are owned by my employer), plus a very wide range of prime and zoom lenses - pretty much everything on the Canon wish list. But for walk-around photography I often take the two m4/3 cameras, the 20 and the 45, and just go looking for photos. They weigh a lot less, which is nice on my poor shoulders, plus they are quiet and unobtrusive, so I can shoot candid photos without being noticed. Our graphic designers use the photos without complaint.
So, all that said, my personal thoughts are that the m4/3 system is more complete right now, with a wider range of lenses and accessories. The NEX image quality is outstanding, and the new NEX-7 looks like it will be a nice camera with more controls than the NEX cameras I've handled recently. But I have no complaints about the m4/3 system.