The EOS M has essentially the same image pipeline (sensor / image processor) as the latest digital rebel. You can therefore expect it to produce very high quality images considering the form factor (though not necessarily any better than the competition mirrorless). Image quality should be better than the G series, though similar to the G1X (which has a slightly smaller sensor, but has a lens optimised for that sensor).

The option to produce DSLR quality images will obviously be attractive to those looking for a backup camera to bring abroad or on remote shoots however there are a few factors to be aware of:

• Autofocus. Performance with regular EF lenses wont be good as the M mount glass. You can expect it to be similar to using liveview AF on the 650D, not as quick as a dedicated phase detection sensor.

• Viewfinder. There isn't one, which will make shooting in bright sunlight difficult.

• Manual controls. Also lacking, changing shutter speed / aperture / ISO will all involve taking your eye of the ball and using the rear touchscreen.

Finally there's the price which puts it on a par with the 650D. I think you would have to be seriously concerned about size to forgo the viewfinder and phase detect AF. Replacing your backup rebel with an EOS M will result your camera bag being 277 grams lighter. If you have a couple of lenses in there already you're unlikely to notice. The space saving is slightly more being about 35% of the volume of a rebel. You might be able to sneak an extra (small) lens into your bag.

The other potential advantage to the M is to use where you don't want to draw unnecessary attention to yourself, or to take photos where "professional cameras" are not allowed (you'll just have to find some other way to disguise your 400 f/2.8)