I'm a big fan of the Rollei system, and the square format. Also the colors off the Phase back just seem right on. Kodachrome like, not Ektachrome.
The dp20 isn't the newest, nor the best, but its pretty darn good for the more thoughtful shot. If you are looking for speed, I'd look elsewhere - either the DSLR (fine) or some of the other newer MF digital rigs. The Hy6 Sinar combo is pretty attractive too these days. Others like the Hassy. I recently went with the dp20 to save some money, get the simplicity and color of Phase, and keep to the Rollei gear already in hand. No regrets.
Eric has identified the advantages of the system pretty well - its an extremely well engineered, very serious system that is quite easy to use. The interface with the back works, but isn't as flawless as the Canon, say for example. High ISO on the DSLR is superior... in fact everything is.... until you see the images. The Rollei Schneider/Zeiss lenses are just something else - like the sweet spot of the Leica lens, but just spread over the whole image. I just can't describe it well enough - but they not only don't let you down, they capture the nuance you were thinking of, and get it on to the picture. The 60 mm Curtagon is one of my favorites, but I'm using the 40mm Schneider (which is the same speed and smaller!) for the dp20 rig, getting about the same coverage. Struggling a little bit with the longer lenses, low light levels and hand holding (I seem to shake some more these days), but the rig is very steady and easy to brace. If you are shooting strobe or tripod, then no issue at all.
If its vaguely affordable, and you are interested, try it. Or shoot me a PM and I'll send you a sample. Nothing pretty, but something to look at.
Geoff