I decided to trade my P65+ for the new IQ260, which I just received from Capture Integration on Friday. I've been playing with it on my 645 DF and thought I'd share some of my observations so far.
I don't really have any experience with the IQ1 series, so many of the comments may apply to those backs as well.
I did most of the processing in Lightroom 5 since that's my normal workflow. Also, I've been unable to coax Capture One 7 into outputting a jpeg. It outputs tiffs just fine though - even when I've selected jpeg settings in the process recipe.
Overall image quality: It's hard to tell any difference between the P65+ and the IQ260 at ISO 50. The IQ260 seems to yield a slightly brighter file, so its base ISO may be just a touch higher than the P65+.
The LCD screen is a lot brighter than the P65+. I've turned it down to 20% brightness, but it's still rather bright. I'll have to recalibrate my eyes and/or pay more attention to the histogram for a while.
The WiFi seems to work OK with a 1st generation iPad, though screen refreshes are sometimes pretty slow.
Battery life seems terribly short compared the P65+, even with WiFi turned off. I only have two batteries, so I may be in the market for one or two more.
The power button is very sensitive, and a quick tap will turn the back off - even if it's doing a dark frame or still writing to the card. Obviously, you loose that frame if you accidently bump the power button. I prefer the P65+ arrangement where you have to momentarily hold the power button to turn off the back. I hope this gets addressed in a firmware upgrade.
Long exposure mode: Phase One suggest using LE mode for anything longer than 10 seconds. In my quick tests, I'm seeing some image degradation when using ISO 140 LE compared to ISO 50. My most recent test was a one minute exposure at each ISO, adjusting the aperture to compensate for the ISO. The ISO 50 shot was much crisper, but showed a fair amount of single pixel noise (though perhaps slightly less so than the P65+). The ISO 140 LE frame didn't show any single pixel noise, but was softer overall.
My usual approach with single pixel noise is to run Photoshop's Dust and Scratches filter at 1 or 2 pixels. I get the best result using a 1 pixel radius and manually healing the remaining spots. Comparing the results, to the ISO 140 LE frame, 1 pixel radius was still slightly crisper, but left a few spots, a 2 pixel radius eliminated all of the single pixel noise, but the result was slightly softer than the ISO 140 LE frame.
My conclusion for now - I'll probably stick to ISO 50 for less than 60 seconds and plan to do some extra processing to clean up the file.
I also noticed some color shift when going from ISO 50 to ISO 140 LE, especially in the shadows.
Lastly - there seems to be a bug in the communication between the camera and back. I'm using the default 60 seconds for the LCD to shut off. When using ISO 140 LE, if the back goes to sleep during the dark frame, the camera will allow you to start another exposure before the back's ready for it. The result is corrupted colors in the 1st frame, and you never get the 2nd frame. I hope that can be fixed in the back's firmware since Phase One stated that they've abandoned firmware updates for the DF body. For now, I'll need to make sure the dark frame is fully complete before starting another exposure.