If I had to choose one camera it would probably have to be the GH1 which is easily the best compromise I have experienced for everything from image quality, to size, handling, responsiveness, and features.
I second to Nic, specially on his concluding line. GH1 is the one, I am using most and a very happy with its output and handeling. The controls are superior to any APSC I experienced before and C-AF on these cameras is simply addictive. I used D90 and loved, then switched to GH1 and later also bought Pen E-P1. Sold my D90, in favor of GH1 handeling. D90 and D80 were my favourite APSC over handeling and I always hated Canon for their very tiny cameras. m4/3 are even smaller but still handle so good in my hands.
Recently I bought D5100 for all its excellent colors, low noise and superb DR. I love its file output but handeling was so wierd. I took both bodies to Norway and later to Italy and Paris. Though D5100 was carrying more useful lens 16-85, I found myself using Gh1 more and then very recently when I went to Lyon, France and Jerusalem, Bethlehem; I left D5100 at home and when I came back, I sold D5100 and said final good bye to APSC.
So I came from DSLRs (Canon 300D, Nikon D80, D90, Fuji S5Pro, Sigma SD14) and then chose M4/3. I had M4/3 and then got DSLR (D5100) and then again dump it. Now, I know APS are bigger sensors and better in few ways in terms of IQ, but so is Film. The reason we chose Digital world is convineance, handeling, operations and mean to reach result quickly and efficiently. M4/3 is a step next in that direction.
I beleive the reason why I stick to m4/3 (maybe later to Nikon CX who knows, in 5 years time) is handeling, Live View, size, maturity, sharpness and good enough general IQ! APS lacks every respect but IQ.
I will still wait for a smaller body fullframe for highest possible IQ with 35mm prime, just to keep it as a backup camera
))