I've found c-pols on my FF 20mm managable. The main reason I bought into the m4/3rds was to play with video, so the filters I would want to use are a circular polarizer and neutral density as the shutter speed is set at 1 / (frame rate) & ISO only goes so far.
That leads to this question: With all the gimmicks like auto HDR & sweep panoramic, why can't camera designers implement low ISOs? The GH2 in video has a floor of 160. Why can't it go to 100, 50, 25, 12, 6 etc. ? Speaking of which, why can't any camera maker do that? Would greatly reduce the need for ND filters.