Seb Farges tested a few C-mount lenses on the Panasonic GH1. The video on top shows the C-mount Canon zoom 25-100mm f/1.8 lens performance. The lens is vignetting between 25 and 50mm and it’s also very heavy. He posted a second daylight video here: http://vimeo.com/16156795.
Click here to see current 25-100mm f/1.8 auctions on eBay.
And he also tested the C-mount 17-102mm f/2.0 (See video below). There seems to be less Vignetting on that lens.
Click here to see current 17-102mm f/2.0 auctions on eBay.
There are many other C-mount zoom lenses. If you tested some of them feel free to share your image/video samples! Thanks!