The first thing I noticed on opening the box is that the GF1 seems smaller than the E-P1. Total volume is probably pretty close but the difference in width really makes it seem smaller.
The build quality feels better to me than the G1 and I think it is the metal body that gives that visual/psychological perspective. For some reasons the buttons feel better than the ones on the G1 that felt very tiny. Compared to the E-P1 I can't say that the difference is better or worse, just different.
The LCD is an area that is a mixed bag. While the GF1 has a higher resolution screen, it is not as bright and vibrant as the E-P1's screen and in bright sunlight where you are shooting toward the light both screens are really unusable but the GF1's is considerably worse. Part of the problem is the contrast on the GF1 screen is very low and the colors look pale and washed out in comparison to the E-P1's screen. I cranked up the GF1's contrast to max to give the images some color depth. On the GF1 the LCD is not recessed meaning you really need to put a screen protector on it (I put one on the E-P1 too as it isn't very recessed either). One area where the GF1 LCD did much better than the E-P1 is in low light. On the E-P1 the screen becomes very noisy and the colors wash out while on the GF1 in the same lighting (same lens on both cameras) the GF1 screen was vibrant, bright and almost noise free.
Focus speed with 20mm f1.7 and 17mm f2.8. I couldn't accurately time with a stop watch but my estimation using a 1,000 One count is that the GF1 is about twice as fast as the E-P1 with either lens. It seemed that the 20mm Panasonic lens was a little faster but we are talking small amounts in the same low light such that at ISO 200 and f2.8 the shutter speed was 4. In brighter light both cameras get faster. I can't say that the focus speed is so different as to make one unusable compared to the other. In very low light the focus assist light on the GF1 comes to the rescue and it is absent from the E-P1. In these circumstances, a virtually dark room, the E-P1 cannot find focus and the GF1 does very quickly.
White balance. The E-P1 wins this one hands down. I had to adjust the WB settings for every shot done in AWB on the GF1. I could minimize the amount of adjustment if I selected a preset that made the image appear to be close but with the E-P1 the color was so close as shot that I didn't need to mess with it unless I was being super critical. Panasonic could learn something from Olympus on this aspect. Maybe by the next firmware release (currently version 1.0 for both body and lens) we will see a major improvement.
Write speed and speed to preview. The E-P1 takes about 6-7 sec before you can hit the play button and look at and zoom your image (you have to wait for the blinking write light to stop). With the GF1 it is almost instantaneous.
Exposure. This is a tricky area and it is clear that Olympus is pulling a few tricks here with their exposure curve. I set up a tripod and mounted each camera on it and took a series of pictures from ISO 100 to 3200 using the same 20mm lens wide open on both. The first go around I used spot focus and spot exposure. I used the camera viewfinder cross hairs to focus on the same spot in the image with each camera and to frame the shots as close as possible. Lighting conditions were identical and all shots were taken within minutes of one another. WB was set to auto on the first round and later I changed the WB on the GF1 to get closer to what I was seeing with my eyes. On the first series of tests, I got the following values:
3 200 10 200
6 400 20 400
13 800 40 800
25 1600 80 1600
50 3200 160 3200
When I looked at the histograms, the E-P1 files were all over exposed to the right.
After I changed to area exposure on each camera things changed again. Finally I decided to see what happened if I set the GF1 to read the scene and then set both cameras to shoot the same shutter speed and same f-stop at each ISO setting. With both cameras, over exposing to the right and then bringing it down in post processing results in much cleaner images. The JPGs on the E-P1 are generally superior to the GF1 and on the GF1 I had to switch to vivid mode while I shot on natural on the E-P1. As you can see if the shutter speed and f-stop are equal at ISO 32 the E-P1 just barely ekes past the GF1 in IQ performance in raw in some cases and loses in others. All of the comparison images are in LR at 1:1.
I then shot some hand helds where I used the flash on the GF1 and no flash on the E-P1. In most cases the E-P1 images were much sharper and the GF1, though shot at a faster shutter speed showed image blur and it took me trying multiple shots before I would get a sharp one on the GF1 unless I forced the shutter speed up to over 100 with the flash. The in body IS is definitely a BIG plus for the E-P1. The E-P1 does tend to blow out the highlights as you can see in the outdoor shots. Also, note when set to the same shutter speed, the E-P1 tends to under expose the image thus making it actually noisier.
I will do some more testing tomorrow and post the results.