Bad video quality on 600D
Should be mentioned that the 600D isn't exactly the best DSLR for video out there. The artifacts that you are experiencing in your footage are the result of high compression. Check your video settings if you can increase the bitrate or choose a different codec other than h.264/AVC if Canon offers one by now for their DSLRs.

This thread might be interesting to read: http://forums.creativecow.net/thread/280/8433

Also a possibility would be to install Magic Lantern on your camera. This enables HDMI output which you could capture (will have a LOT better quality than h.264 encoded footage) and you should have more options about the video codec. On the Canon 5D Mark II they even enabled raw video capture to the CF card but I'm not sure if this is available for other models yet.
Though apparently you loose audio capture at the moment, which might be neglect-able as the audio capture on DSLRs is usually pretty bad. If you capture with HDMI then you can use an HDMI audio injector which aren't expensive so you don't end up with a separate audio and video file that you have to synchronize.