Let me explain this situation I find myself in. Imagine 2 cameras neither of them are bad, but neither of them suit you completely. That's my current situation. I'm looking for a Micro 4/3rds camera to carry around everywhere, where my 1D MkiiN can't go. I've had the Olympus PEN for a long time about 3 months, and I just got a Panasonic G1 a week ago. I've been playing with both the cameras side by side for the past few days and have come up with a few points for people who are in a similar situation.
Size and feel
In the size department the Olympus PEN clearly wins. Being nearly perfectly rectangular its profile is a lot smaller than the G1, and even if the G1 was smaller the way you perceive the PEN is that it's a very small camera, just by the way it looks. This doesn't mean the PEN feels cramped though, the button layout is well thought out and overall it's decent to use.
The G1 is larger but on the bright side the feel is very conventional, so if you like how SLR cameras feel then you'll be just at home with this camera. It really just feels like an SLR smallified.
Here is where things start to get dicey. The G1 screen itself essentially blows the PEN screen out of the water then kicks it in the face. First, the screen resolution, the G1 has a higher resolution screen (460k vs 230k). Secondly, outside in daylight the G1 screen is much more viewable than the PEN screen, and the fact that it articulates just brings in a new dimension of shooting. Oh and not to mention the built in electronic viewfinder on the G1 is insanely nice, rendering sharp details and making it ideal for manual focusing, especially with the manual focus assist which zooms into the focus point for more precise focusing (in good daylight of course).
There is one major caveat though. In low light the G1's live view display (on the main screen and viewfinder) really really takes a dive. No matter how nice the screen is if the camera can't keep up with the frames it isn't that effective. This is a small issue if you use just the selection of Micro 4/3rds lenses since you don't need a perfectly steady image to focus, since the camera will still focus fine. This is however an issue if you plan to use manual focus lenses of any kind in low light, or even stopped down. The frame rate slows down so much that it almost looks like a fast slide show as oppose to a live feed, making it very difficult to manual focus.
The PEN screen on the other hand may be lower resolution, less viewable in day light, and overall not as impressive, but it maintains a good frame rate in low light. This makes it vastly more useful if you were to shoot indoors with your micro 4/3rds camera. Both cameras do get noisy in low light, but the detail rendered in the live view on the PEN is good enough to precisely focus.
I'm not going to really go into that because it's easy to read about at DPreview. Over here http://www.dpreview.com/reviews/olympusep1/page18.asp . My views and observations about the noise are pretty well the same. The PEN wins, generally the only issue that comes out is that the PEN's native ISO is 200 and the G1 is 100. This leads to the PEN's ISO 100 looking worse than the G1.