Like HDV Diver, I am a big fan of the hacked GH2. I have tried out one or two dozen versions of the hack since November. Several of the most active members of the hack community have been creating new versions every few days, so it's hard to keep up with them. There have been some issues that I will try to summarize briefly here. The GH2 firmware has a tendency to oversharpen. This resulted in pulsing images at times and cadence issues. Earlier versions of the hack also have had inconsistent frame to frame data rates that caused cadence issues. The latest versions of the hack have overcome these problems. Detail is exceptional and noise, even with high ISO settings, is very fine grained and film like. I have been testing hack versions with an eye towards eventually getting a Nauticam housing for the GH2. My favorite at this point is something called "Natural" and shooting with it in 1080 30P (HBR mode on the GH2). This is a GOP 3 hack so I believe there are still B and P frames and it is not Intra. However, the resolution is exceptional, motion handling is fantastic, and it's nearly as good as the higher bit rate Intra hacks but with an economical average bit rate of 40 mbps.
You can find the Natural hack or setting here (Natural v2.11):
With this hack you have a versatile m43 camera that can shoot stills, macros, and well-stabilized video. The kit lens (14 to 42 or 28 to 84 in 35 mm equivalent focal lengths) is quite good for video and has good OIS. I have the fisheye and the 45 macro from Panasonic, and have been shooting topside telephoto with Canon L lenses and a cheap adaptor. It's an incredible piece of kit for the price. One downside is that you may find some blue banding because the codec is still only 8 bit 4:2:0. I have been told that you can get good corner to corner sharpness with the Nauticam ports (domes and semi domes) if you stop down to F11 to F13. I frequently shoot landscapes stopped down to F18 with great detail and resolution in video mode. Looking forward to getting mine soon. If you are going to use a hack, read up on the threads from the link above (there a number of quirks, one of which is that you always have to move from the next highest ISO to the one you want to use to avoid the ISO bug that causes noise, e.g., when you turn on the camera, set ISO to 200 then move it to 160 - you always have to do this for the cleanest image.)
Here's some telephoto clips I shot in January with an outdated hack that had some issues. Cadence problems you may notice are from using high shutter speeds and problems with the earlier settings. With the GH2 extended teleconversion mode you actually get full 1920 1080 video with a focal length multiplier of 5. Using a 300 lens and 1.4 converter gives you an effective focal length of 2100 mm: