I am not a pro but i use solely M mode. With the help of the built-in light meter, its just offers the greatest amount of flexibility. You dun have to care about expousre compensation. Aperture, shutter and iso all at the tips of your fingers. It just doesnt make sense, for example, in Aperture-priority, with the iso fixed, the camera sets whatever shutter speed with my chosen aperture to maintain the proper exposure (or what the camera thinks). If I need some compensation, I would need to adjust it specifically. M mode simply gives me the flexibility of adjusting the shutter on the fly as well and whatever exposure compensation is needed, I just read off the light meter.