I have the Kodak ProBack on a Contax 645. I looked at the option of the P20 but decided the price differential at the time didn't make sense for my situation. Don't know what P20's are going for these days; depending on which mount you wanted it for, the ProBacks were selling on eBay for around $4500-5000 back in the spring when I picked mine up--and more for the Contax mount (mine), naturally, perhaps because there were fewer of those on offer. I've tracked every ProBack sold on eBay for the last year so I feel confident about those price ranges. There have been no Contax backs on sale there, to my knowledge, for about four months, and slightly more Mamiya than H'blad models.
I find the ProBack's images to be spectacular at ISO 100, good at 200, and pretty noisy at 400 especially for images in lower light levels. I use NeatImage and have found settings that strike a proper balance between noise reduction and important-image-detail reduction! I pretty much stay at ISO 100, and that works fine for me.
I have used it in studio-like settings, where it really excels; as well as on location, where it can be a bit clunky to use but still does the job. I love it for portraiture, architecture, and landscape/cityscape work. I was even dumb enough to take it to an extreme skate park mounted on my Contax 645 with a bazooka-like 350mm lens out front for some action shooting! Not its forte, as it doesn't like to shoot rapidly and it can actually overheat in such settings, but I got some decent images (check my website if interested.) The contax AF is as much a limitation in such situations as is the ProBack; in fairness, not what it was designed for, so I was back to the old manual-focus days.
I'm overall satisfied with the ProBack on the Contax. Would I rather have had a P25 or P45--absolutely. But I was not willing to pay the difference for the P20 over the ProBack.