I did some not very rigidly done tests with the G1 and the 5D using nonL lens but nice primes (28 f/1.8, 50 f/1.4, 85 f/1.8) and the G1 kit lens, 14-45. It was a dull day, grey lifeless skies, etc. I did use a tripod, same settings (knowing that sensor size would affect it but didn't deal with that but kept it in mind when comparing). The scenes had lots of foliage, detail--and I have to say I am a lot more confident of shooting with the G1 now. I feel the G1 shots (forgot to say I shot in RAW and processed in LR 2.2 which I use all the time) need more contrast, clarity and a bit more capture sharpening to begin to bring them to their best--and a reasonable comparison (even really thinking its not a fair comparison), but that's easily done--even presets in LR could be created. I had some areas of very dark shadow and the G1 files were able to have detail brought out without a lot of noise resulting also.
I also have been shooting some test shots indoors with very low light--and surprised what I can get with higher ISOs--I'm used to the 5D so have a fairly high standard. I wish the lens were faster--but then they would be more expensive and larger I assume.
I don't have legacy lenses--surely wish I did--but I'm waiting for the 20 f/1.7 and I may look around for some others that will give me what I like from images. In the meantime, the 14-45 is a very nice lens and the 45-200 can also give some very nice results though I don't feel its as good as the 14-45.
To sum it up---I'd add more contrast, clarity, capture sharpening to any file to get the best out of it. I don't shoot in jpeg so can't compare there.
I neglected to say that using the MF assist is just terrific in many circumstances though the AF works very well. Using MF assist makes it possible to put my focus wherever I want it (as well a the ability to move your AF around with a small change in the settings)--a lot more difficult to do MF focusing with the 5D and I often MF with tilt shift and macro lenses so am used to doing it. Even with an anglefinder C on the 5D--I would say the G1 is much easier.
Diane