I haven't done much works recently with the Proback. The reason has been the type of shooting that I've been doing for the past few months, wildlife and travel rather than landscape. I'll be using it again next month on my next Iceland shoot and workshop.
But, the back remains a favourite and when I need maximum image quality it is my first choice. At ISO 100 or 200 it is superb, and I've never had any regrets other than the fact that it's not full-frame.
There are new backs coming, such as the Phase P25, which will likely be very tempting, but also mightily expensive.
The thing to realize here is that at the top of the food chain, which is where these products are located, the incremental improvements are very small indeed. A used DCS Pro Back for under $7,000 (referbished by Kodak) would be a very good investment and I can recommend it highly.