I used a Hasselblad V for about 20 years, thought the future was 645 so moved to the Contax 645 for about two years (using every lens including the 350mm) with film and with a Kodak pro-back, then moved to the Hasselblad H for a year with film and a Phase One back, and finally went back to the Hasselblad V with a Phase One back. I'd be open to moving again if something better for my needs comes along, but to be honest I'm a little tired of reading instruction books by now so it'd have to be a really significant improvement!
The Contax 645 was possibly the prettiest camera I've ever used, no small matter if you're handling the thing all day everyday. It had one or two outstanding optics, I'm thinking of the 120mm macro in particular which I miss to this day. And for a portrait photographer say, prizing a fluid and mellifluous transition from in-focus to out-of-focus, virtually all the lenses would be gems.
However, ultimately the camera wasn't for me. Creamy though the lenses are there's a shortfall in ultimate sharpness with some of them. I realise this may ignite the passions of several forum members, so I'd suggest an objective solution. Go to the Zeiss web site and look up their MTF curves and compare the Hasselblad V 350mm superachromat with the Contax 350mm; or go to Hasselblad's site (they use Zeiss MTF equipment with exactly the same test protocols as Zeiss so it's a rare case where MTF comparisons between manufacturers is valid) and compare the Contax 140mm and 210 lenses with similar H series optics.
Also the viewfinder on the Contax was simply darker than both the Hasselblad alternatives, I eventually found a third party Contax screen option that helped, but at an incredible price. Battery power was always a problem with the Contax, there's a larger battery pack accessory that helps, but it also turns a silk purse into a sows ear. And finally there's the Contax auto focus, which was painfully slow and halting in poor light, the H system auto focus is just so much superior, and the V system may be manual focus but you get the screen brightness and focusing aids necessary to deal with that. However, if I were a portrait photographer I'd probably still be using the Contax and be singing its praises to the heavens, so I guess it just goes to prove that it's the photographer's own foibles and the needs of the application that count when it comes to choosing a camera!