I'd like to know.., which value for "Video Quality" works good for you guys (84 seems to be the "default"), in which you still get "good quality" videos but...not so "heavy" (MBs-wise) 
I'd like to reduce MBs weight for my videos, but still have a nice quality... 
I don't shoot video much and haven't really experimented with CHDK quality override, but from a purely theoretical point of view the optimal bitrate depends on what you shoot, and especially on the conditions you shoot in.
A scene with lots and lots of fine details and high contrast edges is harder to compress and requires a higher bitrate compared to a scene with not much detail.
When you shoot in dark, there's lots of noise. Noise doesn't compress well, which means that you need a high bitrate and even then the image will be crappy. On a sunny day you can compress more and still get great quality.
Motion does not affect MJPEG compression (which is what our cameras store inside the AVI container), because each frame is compressed independently. In this respect it's easier to handle compared to MPEG-4 used by many competitors (and some new Canon models).
