Thanks for the post.
The EOS M will work with the same EX-Series Speedlite flashes that the EOS Digital SLRs do.
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BTW - it Canon's model numbering system for flash, if you remove the trailing zero from the model number, what remains is the "guide number" of the flash as measured in meters. E.g. a 320EX has a guide number of 32. A 430EX II has a guide number of 43, etc. The 90EX has a guide number of 9.
The guide number (and they can be measured in either meters or feet but Canon uses meters) is base value which tells you how far the flash can be from a subject and provide adequate light for an exposure BUT... since the camera's ISO setting and f-stop would affect that (shutter speed does not affect it, btw) the guide number system assumes a base of ISO 100 and aperture of f/1.0.
The ISO 100 is no problem... every camera can do that. But the f/1.0 isn't possible (Canon used to make an f/1.0 lens but hasn't made it in years). The reason the guide number system uses f/1.0 is because it makes the math VERY easy to find your true distance. Just divide the guide number by your ACTUAL f-stop. e.g. if you are shooting at f/5.6 then you'd divide the guide number by 5.6 and that's your real distance.
This sounds like a lot of power, but keep in mind that when you bounce a flash (for better light) you not only increase the distance substantially... you also lose a lot of light that wont reflect. Even if you run the flash through a light modifier, most modifiers eat a bit of a light (1 stop is very common.)
For this reason, you usually want to err on the high side...
A 270EX II is good for bouncing but only if the subject is very close and the ceiling is low (small rooms). A 430EX II, on the other hand, is much better for subjects a bit farther away and larger rooms.
On the 270 and 320 flashes, the head slides forward or back manually to re-shape the reflector and this determines whether you'll have a wide (but shorter range) or narrow (but longer distance) flash coverage. On the 430EX II, the reflector is motorized and the flash automatically moves the reflector inside the head to match the focal length of the lens.
The 320 and 430 can be used as off-camera flashes (remotely triggered by an on-camera "master" flash or one via Canon's dedicated Speedlite ST-E2 commander. The 90EX can actually be used as a "master" flash to trigger off-camera remotes.