I own and use both Medium Format Digital and several DSLR's. We often use both on the same job, each one has its inherent advantages. Sometimes we choose just one system depending on the brief, but usually we use both.
For me the super high flash sync speed of the PhaseOne IQ backs is critical, I often shoot outdoors and this gives me great flexibility. Are DSLRs as good as Medium Format ? This would all depend on what you need/want the Camera to do? Sometimes a DSLR is a better choice, oftentimes for me though it's medium format, but this is a consequence of how I like to shoot. I also love the depth, look and feel of Medium Format.
We did a shoot a few months ago at a large TV station here in London, after doing the scout I was certain I'd shoot it all on my Nikons. When we were prepping the day before the shoot, I suggested to my assistant to throw the Phase case in, just in case. On arriving at the location, we set up for the first shot on the DSLR and I just wasn't 100% happy with how the image looked, so I tried the Phase and straight away the shot was right. I know this sound ridiculous, I was shocked. I really thought we'd use the Nikon. The DSLR shot was very good, but the MF was better. We ended up shooting the whole job with my P30+ back on the Phase One DF+ camera. Why the P30+ ? because this has great files at 800iso, we now also have the IQ180 (this replaced our P65+) and I'm using the sensor plus feature at 20mp's for those times when I don't need an 80mp file.
Last Wednesday we did a magazine shoot with Donovan Bailey (gold medal Atlanta Olympics 100metres, was the fastest man on the planet) Guess what? We shot the DPS and holding shots with MF and the reportage images with DSLR.
Look carefully at what you are doing and how you are doing it, this is a business decision.
The argument for and against MF isn't just about resolution, for me the look and feel of MF is often preferred. I think it makes getting the shot easier too, when I want something special.