I have now had the opportunity to use the E-P1 for a couple of week including on a recent trip to San Antonio. Here is what I have found so far:
1. I miss the viewfinder of the G1. In bright sunlight or in dim light the EVF is superior to the LCD.
2. While the articulating LCD of the G1 is nice, the viewing angle visibility of the E-P1 is sufficient that I didn't miss that as much as I thought I would.
3. The G1 focusing is definitely much faster but I found it to be a little less accurate. What I mean by that is using spot focusing (the green rectangle as the object to be in focus) and listening for the confirming beep and then shooting, I found G1 images that were out of focus and I have not seen that on the E-P1.
4. Size comparison, the E-P1 just feels smaller and with the leather half case and strap (see my images in other threads) it is much easier to carry around and almost feels like its not there. I have only carried it with the 17mm pancake as a 34mm effective walk around is about a perfect combo. I have the 14-42 but I haven't even used it. There is a big difference between carrying the E-P1 and an M8, even with the M8 and 28MM f2.8 lens. The M8 is much larger and heavier and you always know it is there.
5. Highlight exposure. The E-P1 blows highlights very easily and in some cases cannot be overcome. For example I was trying to shoot moon shots last night at a full moon. No setting I could find would allow me to show any moon detail (i.e. man in the moon). All I could get was a white sphere. Of course, this could be due to my lack of skill but I have seen G1 shots that showed a great deal of detail.
6. E-P1 high ISO noise is better than the G1, especially above ISO 800 where the G1 is basically unusable. The E-P1 can produce usable images up to ISO 3200 and the noise looks more like film grain than large blobs.
7. Wife Factor. The E-P1 wins this one hands down. My wife was with me and never complained about my having the camera with us or taking pictures with it. With the G1 it was always do you have to take that big ugly thing with us?
8. Video-since the G1 lacks video it can't compare to the E-P1 and the video is surprisingly good. However, it does blow the highlights very badly. Sound quality and video quality are very good. Part of this may be due to my lack of experience and knowing how to adjust for exposure while shooting video.
9. The following is purely subjective but I seem to get more keepers with the E-P1 than the G1 but I'm not sure why.
10. Ergonomically from a usage standpoint, the G1 is a better design. Having more mechanical controls rather than menu button pushes is a big plus and the MF is superior. I find the built in IS to be preferable to the in lens IS as it makes my old Leica and other lenses IS lenses and I can get very good low ISO shots hand held in near dark conditions.
11. Weighing all of these factors for me I have decided to keep the E-P1 and I am still selling the G1 (I had it sold on Amazon but the buyer wouldn't respond to my emails so I cancelled the sale, something didn't smell right). They are both very good cameras and if the E-P1 had a viewfinder, in spite of its slower focus it would win easily.
I will post some images with explanation for examples of what I mean.