I also have a Nikon D90 (with a Tamron 18-270). How does the image quality of the GH1 compare to the D90?
This was by no means a scientific test of mine, just my impressions after some shooting with both the 14-140 and the 18-200.
The 35mm-equivalent for both lenses are 28-280 (10-fold) and 27-300 (11.1-fold).
However, with a little bit of cheating (using 16:9 for wideangle and 1:1 aspect ratio for tele with the GH1) I have at least the same range as the Nikon-lens.
Distortion on the Panny is much, much less than the Nikon.
Vignetting on the Panny at wideangle is much less the Nikon.
No zoom-creep on the Panny, but on the Nikon.
DR on the Nikon is about 1 stop better than the Panny.
Nikon has more RAW-headroom than the Panny.
Nikon VR is about 1 stop better than Panny OIS.
ADL on Nikon works better than Panny Intelligent Exposure Adjustment.
Default Color on the Nikon looks better (but can be tweaked in the Panny to look alike).
Default sharpness on the Panny looks better (but can be tweaked in the Nikon to look alike).
Nikon AF is faster.
Panny AF is more precise.
Nikon flash system is much better than Panny.
The EVF is fantastic.
The swivel LCD had been on my wishlist for a long time.
All this was not measured, these are just my impressions. YMMV.
I´m not a brand-loyal person, I´ve been using whatever suited my needs over the last 35 years. And I needed different types of equipment for different phases of my life. So I used Canon, Pentax, Minolta, Canon again and now Nikon.
But for my personal type of shooting the GH1 fulfills all my needs, and that in a smaller and lighter package. I´m just waiting for a fast prime.