Just to round this up after a few weeks with the G12, this is what I've learned:
Negatives (but only just)
* the G12 isn't an SLR and shouldn't be expected to perform as such. Image quality is noticeably inferior, but still a long, long way better than what I expected from a compact and certainly I've never thought, "this shot is not worth taking because the IQ won't be good enough"
* my biggest miss is that the maximum aperture of f/8 makes it difficult to take a shot with a foreground object and a background all in good focus and making full use of the frame. However, it just means you have to think about your photography a different way. I can usually get what I want by standing further back and cropping the shot in PP. I guess with a small lens and small sensor, there quickly comes a point where diffraction ruins the image if apertures are pushed further than f/8.
Positives
* it's an unbelievably accomplished camera for such a small size. I read all the moans about it being bigger than you'd expect from a compact; I can only assume that those are from people who have never had to drag around SLR kit, and whose primary concern is to be able to get the camera to a bar and take pictures of their friends 
* RAW mode is excellent; detail recovery and the like aren't far off what I expected from my 30D.
* controls are phenomenally good; it sometimes feels like using an SLR now I'm into the manual mode routine of exposure lock with * -> compose picture -> obtain focus and make adjustments with wonderful separate aperture/shutter/ISO wheels -> take shot. Even my 30D didn't have a wheel for ISO or exposure compensation..
* video quality is excellent, for anybody who needs that kind of thing
* did I mention how good it is for such a small size? It fits on my belt, or on the front straps of my pack. I used to have to stop and take my pack off every time I wanted to take a picture, for heaven's sakes 
Cheers to everyone who helped with this thread
I think I made the right choice for me, and I look forward to continuing to pollute the Best-With-A-G12 thread for many years to come, even when we're onto the G16..
Finest moments after three weeks (mostly posted in other threads, apologies if you've seen them before):

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