Prices of digital backs have come down and you can now get one for around £5,000 in the UK, even cheaper second hand.
Comparing the results between a 16mp digital back on an old 1980's Hasselblad C/M and a new pro full frame Canon DSLR is an eye opener. The quality from the Hasselblad is better. Images are super sharp, files stand up to interpolation better, there's more shadow and highlight detail, colours are spot on and the files are true 16bit from RAW. DSLRs don't actually generate true 16 bit RAW images.
One way to try a digital back on your medium format or for that matter large format camera is to hire the appropriate back. It's not cheap but if you incorporate it in your costs for the shoot on the day then the extra quality could well benefit your work and give you an edge that clients will appreciate (if you're a pro).
Working with medium format is slower. My old Hasselblad C/M doesn't have a lightmeter, no autofocus, everything is manual. I don't even use a pentaprism so the viewfinder image is reversed. This means taking a more considered and slower approach which is not a bad thing at all.
Coincidentally a professional photography magazine in the UK has just done an article on the same subject and their writers were raving about the results. They hired a back from the Pro Centre in London. The magazine remarked that for faster work pro photographers could settle for a D300 or a D40, to save some cash and use the digital back solution for super high quality stuff in the studio and outdoors.
No doubt prices will continue to fall as many pro photographers move up to the larger mp backs available today.
One thing to bear in mind is that there is a crop factor so for example the Hasselblad's standard 80mm lens becomes a 120mm. If you like shooting very wide angle then you will have to consider the implications.
I'm glad I hung onto my old Hasselblad gear now and I may dust off the Mamiya Pro 645 too. We'll see.