I just switched from Nikon (D200) to Canon (1Ds mkII) after seriously contemplating medium format digital. I looked long and hard at the image quality from various options, including digital backs, the forthcoming ZD, Canon and Nikon (everything except the Pentax, which nobody's seen samples from yet). I'm going to be shooting a lot of landscape this summer for my PhD, and couldn't afford to wait for mythical cameras from Canon and Nikon, which might show up this winter (especially with Nikon's disappointing "announcement" on full frame)... The 1Ds mkII has superb image quality (judging from my first 500 shots), including more resolution than its already impressive specifications suggest - it seems to have much more detail per pixel than the D200, and quite a bit of extra dynamic range (again, compared to a D200).
I'm very satisfied with the camera so far, and have already arranged to sell my Nikon system. The one thing I wish Canon would make is their own version of a D200. A reasonable weight, rugged camera with a 1.5x (in Canon's case, probably 1.6x) crop... For 80% of my work (landscape and macro), the 1Ds mkII is wonderful, and its weight is worth it. The full frame is a perfect fit for that work. I'd like to complement it with a cropped sensor camera with a really high pixel density for wildlife, though. Wildlife shooting is always focal length limited, and the same full frame I love for landscape becomes a curse for wildlife. Canon doesn't seem to offer the cropped body I want, though. They have the 30D, which is a D80 competitor, and a bunch of Rebels which don't fit big hands or stand up to rough use. Why can't they make a D200 competitor? With the D200 already over a year old, I would think Canon would have come out with something to embarass Nikon by now...
What I'd love to see:
12 MP with noise and DR capabilities equivalent to 30D
1.6x crop
Improved AF (at least D200 level, maybe better - doesn't have to reach EOS-1 level)
Weathersealed, rugged body, at least to D200 standards, if not to EOS-1 standards
Built-in remote flash control (Nikon does this in the D200 AND D80) - I'd probably use this for people as well as wildlife, and there are times where remote flash is really nice, ideally without bothering with a ST-E2).
Other than the 12 mp, this is nothing more than a D200 with an EF mount on it! If Canon made one of these and sold it for, say, $1600, how many folks here would buy it? It would seem to be a great complement to a 1Ds mkII or a 5D.
-Dan