As for IQ, for my purposes M4/3 is good enough for anything I shoot for the newspaper. So, that's my "good enough" standard. I carry a 1DMkIV, though, for reasons that have nothing to do with IQ (like durability, killer fast AF, 10 FPS, 12,000 ISO, 300mm 2.8 lenses...blah, blah, blah).
So, for me professionally, ultimate image quality is less a factor than versatility. Personally, I love extremely sharp, noiseless (grainless) photos. You take your choice and you pay your price.