Great camera but:
1) you can get the same sensor in a truly shirt pocketable case, in
the S95 (now superceded by the S100) The size of the S95 is very
similar to your 5900...the G12 is quite a lot bigger
2) the G12 may need you to up your budget a little (as would the s100)
The lens is the same at the wideangle end but not as long at the
longest setting on the G12. Either are wider at 28mm than your 5900.
It is also a slighty faster lens and does shoot RAW. The S95 is very
controllable although perhaps not as easily as the G12 in that it has
less buttons/dials. However, it is easy to configure your favourite
couple of adjustments to the main control rings. The biggest drawback
in comparison to the G12 that kept putting me off before purchase was
the lack of an optical viewfinder - I REALLY did not like that
idea...but the smaller size has more than compensated for this. The
screen is actually quite useable - even in bright sunlight as long as
I remember not to use my polarising sunglasses! The optical viewfinder
on the G12 isn't great either if you're used to a dSLR .
I now find that when using my Canon 50D, when I bother carrying it, I
look for the image on the screen rather than through the viewfinder!
Don't rule it out without handling one.
Jessops have some S95's available at £300.
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Ian