I haven't used the omd 1 yet, but I consider 4/3 professional. I know I've made money with them, especially the GH3.
I don't think were at the exact point where physical sensor dimension isn't a factor, but we're close and when I use the m43 cameras I never think of them as cropped anything. I just double the focal length, know what lens to use then shoot.
Whatever is in the viewfinder, lcd screen and computer is all that matters and I've shot my omd side by side th 1dx and in most instances there is not a great deal of difference, for most imagery to me I like the 43 better because their not glass smooth.
In regards to size, the omd1 really is very close to the same size as the omd 5, much smaller than a 5d3, tiny compared to a 1dx, but in a lot of ways more useable, especially with the 5 axis image stablization, price, lens size and the ability to carry a huge kit in a shoulder messenger bag.
My only three gripes with the omd1 is the lack of a better video file to exceed or even match the gh3, headphone sound sampling out and an easier menu system for setting button functions.
My one real upside of the omd 1 is the legacy 43 lenses. If you've ever used those f2 lenses, like the 150 you'll know what I mean.
Oh yea, also the oly always seems oversharpened out of camera, but finding the right preset in c-1, lightroom or the best Irdient Developer will fix that easily and give you a look that makes you wonder why a 1dx costs 6 grand.
Now I guess where the rubber meets the road, will I buy it? Maybe, probably, but next will be the blackmagic 4/3 video camera (if they solve the battery issue and can format cards in camera) because the footage I've seen from that camera is very pretty and ready to edit right out of camera.
Personally though, I've never seen a more capable camera than the gh3's for the money, but olympus has a history and some style to it that the panasonic's lack. Actually it's in the top sale of Amazon cameras today, the first time every for a olympus camera and one of the highest priced in the top 100.
Maybe more people have been anticipating it than the naysayers thought.
My suggestions to anyone wanting to try 43 is to rent one and put it in a paying gig. I think lens rentals or borrow lens will rent them. The oly 43 primes and the leica 43 25mm are to be seen to understand how sharp and good they are.