I've had a G1, kit zoom, and 20mm pancake, for only about a week now -- and I'm amazed at the quality. I've owned an original 5D for about 4 years+, and a 5DII for over a year, using tested, top quality "L" lenses. I'm pretty confident that a G1 file, exposed on a tripod with cable release, at base iso of 100 in RAW, will print as well, pixel per pixel as an original 5D file. Where the 5D will surpass the G1 is at iso 400 and above, thought the G1 is still excellent up to iso 800.
The G1 nails focus dead-on, in my experience, much more often than does the 5D; I think the contrast detect AF may be more precise than the Canon system? Whatever, I'm amazed at the quality of these cameras and lenses, and then when you take into account the prices and light weight and portability the Panasonics are amazing.