I think most guys bought they backs after their Canon because it did not meet their expectations. :-)
Actually, Canon Always met my expectations, from the 1ds on.
The only glitch was the 1ds2 which mine and others had a file corruption problem with shooting then quick lcd viewing.
Canon eventually slowed the cameras up and it worked.
I had two reasons for going medium format. First was when the 1ds2 hit problems I walked into Samy's with the intention of buying a new camera. The Hasselblad wasn't there yet, the Aptus wasn't out, Phase had no LCD on their backs so I bought a d2x and a ton of the last Contax in the box cameras, a leaf Valeo (which was a great back) and moved from the Valeo to the Aptus 22.
The second reason for medium format was at this early stage in digital there was a lot of "digital experts" pitching every project I did all saying their medium format backs smoked those Canons, so since I had two digital backs and we knew our stuff we could get the "experts" out of the room and the billing, but honestly, if the 1ds2 hadn't flaked and the digital experts had not appeared I'd probably have never owned any still camera but a Canon.
I've also owned the D2x, D700, D3 (for low light) and a bunch of nikon glass from legacy manual (which I used in motion imagery) to the newest Nano coating lenses. My favorite Nikon lens is the old 50mm 1.2 manual and the newer 200mm f2. I love the 200.
In regards to medium format if I was in 2007 I'd think about buying a new Leaf of Hasselblad, though we shoot nearly every project in motion and stills with continuous light and medium format isn't the world's best camera for that scenario.
Also after some brief thought this year when I looked at Hasselblad, Leaf and Phase (in that order), I decided I wasn't gaining enough over my p30, p21 and contax to make the investment, but what really happened since 2007 is the cameras didn't change, our business model did.
Today our medium format style cameras are RED motions cameras have two R1's and a Scarlet. They are the medium format equivalent of motion imagery. They require a personalized Rep to keep up with firmware, changes and software upgrades, they require crafted light and they're damn expensive, at least in the photo world (not the motion picture world).
I'm sure they will al be replaced someday with something smaller and cheaper.
But today, I don't think Canon can be ruled out. I'm sure they'll have a 36 to 40mpx camera and the internet blogs will light up in comparison tests, medium format guys dogging it, the same thing we've seen forever, though in reality, motion or still cameras are made to allow beautiful images to be made and the pundits will hate it, but when Canon comes rolling out, I'm sure a lot of eos lens owners will buy and buy and buy.
I kind of thought the 1dx was the last semi expensive still camera I would ever buy, but with the 1dc just looking at what it can be, not just what it is, if they take it up a notch. go with in camera stabilization, long run time 4k to 5k and allow it to produce a high rez still, Canon could be back on top.
IMO
BC
Shot with the original 1ds which I still use from time to time.