I shoot with canon 1 series and L glass equipment for the majority of my photos. However for portraits, nighttime city scapes and some travel imagery i still shoot with my hasselblad system. The hasselblad produces a quality that is really truly amazing.
If you currently have a hasselblad system you can buy a brand new Imacon back for it from Hassleblad for about £5000 or about £3400 2nd hand. I think that if you have the 500 series body and lenses already then this is the best way to go.
If however you are starting from scratch then buy a 5D and some L glass and be happy.
Chris