The G12, G15, & G1 X are all "advanced" point & shoots. They're actually designed for people who already own DSLRs but need a point & shoot for situations where bringing a DSLR is either impractical or simply not permitted.
For example... I own a 5D II. If I go to a concert event then I might like to bring a camera. Most venues wont let you bring a "professional" camera -- and consider any camera with a "removable lens" to qualify as a "professional" camera (ok, so clearly not the same standards as the rest of us might use, but it's their venue and they make the rules.) Often times I have a connection to an artist and can get permission... but not always. SO... I bought the G1 X because the lens isn't removable (it has a sensor size which is 80% the size of an APS-C DSLR... for a point & shoot that's actually huge.) I don't have any firsthand experience with the G15 other than the fact that the body styles and layouts are extremely similar (technically the G1 X is a physically larger body... they had to grow the body size to make the sensor as large as it is.)
I can tell you that I DO love being able to use speedlites with the camera and only these high-end models have a dedicated flash hot-shoe. I can also tell you that the image quality is extremely good. If I take side-by-side shots against the DSLR and compare them it's actually hard to tell the differences PROVIDED that I limit the DSLR to lens focal lengths and exposure settings that matched what the G1 X could use and also similar crop the images to match the G1 X. Since it uses phase-detect AF, there is a slight delay in shooting (as all point & shoots have.) That means I would NOT recommend one for sports or action. Basically it isn't quite as good as a real DSLR, but I'd recommend it for uses where you either can't or shouldn't have a DSLR with you.