I have a G12 and a 7D with a pack full of zoom lenses. I bought the Panasonic Lumix LX5 and returned it to get the G12. My room mate has the Canon S100 and I've played with it a bit.
For my uses and subjects I prefer the G12 for general purpose. If I wasn't interested in an auxiliary flash then I would opt for the S100 because it does everything the G12 does (through menus instead of dials) but seems to be about 1/2 the size and fits in a pocket so much better. My G12 always finds a spot in my pack whether it's a slim slackcountry pack or my full touring kit and I have pulled it out in many more places than my 7D. By the time I have my 7D and 3 lenses in a padded bag it adds ~20lbs to my pack. I've done long days with it but it's not fun.
I really only had 2 issues with the LX5: I was already familiar with Canon menus and controls and I already had Canon flashes that are compatible with the G series.
There are extension tubes available for the G12 if you want to play with a different field of view than the standard zoom range but you lose image quality.
The G is slow to pick a focus point, slower to change manual controls, and shoots at a slower frame rate than the full size body. If I'm serious about shooting action (snowboarding and skiing for me) then I take out the big body. I've I'm trying to document my adventures and shoot a few pictures of rocks, trees, skin tracks, neighboring mountains, etc then I leave the heavy equipment at home.
The small sensor size is also a benefit for shooting landscapes and low light subjects because the depth of field at F/4.0 is much deeper with a small sensor than with a big sensor so I can shoot in low light at F/4.0 at a low ISO and a higher shutter speed for decent images with very forgiving DOF for landscapes. It makes the artsy, selective focus shots all but impossible though (do it in post if you have to).
