The Contax 645 is my main camera and I'm very interested in the Phase One IQ series as a first digital back. I've read all the official documentation, but I'm curious to hear about your real-world experiences with this combo. Anything that surprised you? Weird quirks? A particular lens that looks better than it did on film? Buyer's remorse? Seller's remorse? Anything!
Actually the biggest news here is that there really isn't any news.
Shooting a Contax with an IQ is like shooting film but with an iPhone on the back of the camera.
If you're using the IQ140 there is a crop from the full frame which does effect the look and DOF characteristics of each lens.
You'll also have significantly more detail than with film (especially with the IQ160/260/180/280). You don't HAVE to get 100% sharp images with every shutter release (if you have a bit of camera shake for instance you might only get as much actual detail as you did with film), but if that's a goal of yours (e.g. you want to print big and get the most out of the resolution of the system) then you'll need a bit tighter technique than with film (e.g. bit faster shutter speeds when handholding).
And of course you'll have very robust raw files instantly. Beats the hell out of scanning IMO.
As far as camera operation goes, the Contax is one of the least-changed cameras to shoot with when you put a digital back on it. You could literally put gaffe tape over the LCD, disable the "beep" ready-sound on the digital back and you'd be hard pressed to find any difference to shooting film other than you don't run out of film every 16 shots.