All the MFT cameras other the the GH1 crop the top and bottom of the frame to go from 4:3 to 3:2 or 16:9 aspect ratio. This means that in 16:9, while the horizontal angle of view is maintained compared to 4:3, the vertical angle of view is reduced.
The GH1 maintains the diagonal angle of view in 4:3, 3:2, and 16:9. This means that in 16:9, the horizontal angle of view is increased compared to 4:3, and the vertical angle of view is reduced, albeit not to the extent that it would be reduced on other MFT cameras.
The bottom line is that 16:9 on GH1 makes any lens wider than 16:9 on the other MFT cameras, and 16:9 uses more sensor area on GH1 than on other MFT cameras.
As for banding, Nik Dfine does a good job of debanding GH1 images. For example, this is a nominal ISO 3200 (ISO 4176 per DxOmark) GH1 image. Banding is very obvious:
Here it is again after Dfine:
Dfine debanding can result in some odd effects. For example, if you look closely around the edge of the lamp, there is a halo effect inside and outside of the edge. However, it's pretty easy to strike a balance, and any untoward effects are usually much easier to clean up than the banding itself.
Here's another example, this time ISO 1600 (ISO 2154 according to DxOmark data) and pushed a bit in Lightroom. This was shot through thick and dirty glass at a local zoo. Please ignore the reflections, which were unavoidable and do not have anything to do with the GH1 image quality:
After Dfine:
I personally don't often find banding to be an issue with the GH1 (and I bought an early model), and when it is, Dfine almost always does the trick.
As for the original question "Should I buy a GH1 right now", I'd wait and see what the GH2 offers.