With the release of Canon EOS M, the only question I have in mind is how will it perform as a second/alternative/backup body for prefessional/semi-professional photographers? We can use our existing EF mount lenses, also the sensor is APS-C sized. Will it provide better optical performance than Canon G11/G12 or G series cameras in general? Also how does it compare to people who uses FF mainly but have APS-C DSLRs as an alternative body.
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:
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)
The Canon EOS M stands virtually no chance of being a professionally used backup body and I speak strictly in the sense of professional event photography.
The EOS M is certainly an advancement for the compact size yet great quality department, but it simply does not have the demanding feature set that is required in any professional capability. Some significant cons to using the EOS M in a professional capacity include:
Using the Canon M does have some advantages. It is small in size(can be an advantage), can utilize existing EF and EF-S lenses with an adapter, should be quite quiet at shooting, and the price is reasonable.
As I mentioned in a comment elsewhere here, I think that the Canon EOS M's biggest advantage is it's size. It isn't really cheap, it isn't really the best quality(think AF and APS-C), and it isn't full of the most professional features. So why would you choose this as a backup camera? For it's great size advantage? I really don't think so. The only use case I can think of is someone who does a significant amount of destination weddings, where you have to worry about the portability and size of your kit on a whole new level.
Overall, I don't know of any professional that would carry a Canon G11 or a S100 in their bag and consider it a backup, and I don't see that happening with this either just because they can now use the full range of EF lenses. If you are using this as a backup for PERSONAL use, certainly it will capture images and fit the majority of use cases for personal use.