I'm a big fan of m4/3 though probably not my first choice for aviation photography, still possible.
For me the challenges are
1 - lack of viewfinder with many models. The screen is fine (in fact I prefer it when shooting close and wideangle), for some purposes but I find as the focal length increases, and/or action photography its struggles, When I got the 45-175 zoom for my GF1, I soon found that the separate detachable EVF was a must buy.
Happily my OM
-D has both built in - and the EVF is pretty good. Not Canon 5D quality, but probably more useable than the viewfinders on older crop body DSLRs as it has some tricks that they don't, like adjustable brightness. And these get better with each new model.
2 - the poor servo focus performance - it can't track a moving object anything like a DSLR. On the other hand on my OM
-D, static focus is unbelievably fast and accurate which reduces the need for servo mode. And of course you can focus on any visible part of the screen, not just a handful of focus points. For static aircraft, side -on approaches and the like, the AF
will be fine. Fast jets coming toward you - unlikely to be in focus.
I believe we'll be seeing hybrid focus systems in M4
/3 real soon (I think Sony already have them), which should be the best of both worlds.
Another plus for the aviation photog. is you can mount (with adaptors) pretty much any lens ever made, which opens up the possibility of getting hold of old exotic lenses at affordable prices.
Finally, the best camera is the one you have with you when you need it. I don't want to carry my Canon outfit with me wherever I go - too big and heavy. But I have 2 m4/3 bodies, and 5 lenses which don't even fill the bottom compartment of my everyday all purpose Lowepro backback, and together way much less than my Canon 500 f4.
Colin K. Work, Pixstel