I use medium format digital (Mamiya 645 AFDii and Leaf Aptus) alongside 35mm digital (Canon 5D3 and 1Ds2) and a Linhof Technikardan.
I use the MFDB wherever possible, the files that come out of it are beautiful, plenty of potential to push and pull them, sharp as a tack (no AA filter) and beautiful colour rendition.
But, it won't do exposures over 32 seconds (Phase One P45+ is the first to do that), base ISO is 25, max is 400 and is noisy and no liveview.
If I need high ISO I have to use the 5D3. If I want high frame rate I use the 5D3. If I want liveview I use the 5D3. If I need a long exposure I use the 5D3. If I want fast AF I use the 5D3.
The MFDB is a lovely thing to use, when the situation is right. Landscapes in good light it is perfect, whether on the Mamiya for quick jobs, or the Linhof if I've got all day, but it isn't a perfect replacement for a 35mm based system. The IQ250 seems to be heading in the right direction, with proper liveview, long exposures and decent high ISO, but it is £25k and not full frame so wide angles become an issue. Also it isn't recommended for use with wide angle technical cameras due to the microlens issue similar to the P30+