I started with a Rolliecord Vb in high-school - because my father had one, and over the years, I've owned and use a Hasselblad 500c, Pentax 67 & 645, Mamiya RB67, Bronica G1, Fujifilm GX680, Kiev-88, Pentacon Six, Yashica & Ricoh TLRs, and various non-SLRs (Mamiya 6 & 7s, Fujifilm GW690s and GS645, etc...). I currently use a Hasselblad 500cx and a 200fe.
What Sirius said above is correct, of the systems I've used, the Hasselblad is one of the best in multiple categories. And there are still a lot of working copies around, which speaks to their longevity and popularity.
- They were the first system MF camera with full interchangeable lenses, backs and finders.
- They were/are the smallest most compact 6x6 SLR made - idea for field use
- They had full leaf shutter synchronization for electronic flash as all speeds - something very important in the professional studio.
- They were reliable and had a professional service program
- Lenses were sharp and contrasty
Arguably, the Bronica SQ was the closest competition, as it did everything the Hasselblad did (including sharp lenses). And indeed, it was extremely popular with the Portrait/Wedding professionals as they cost less, were lighter to use hand held, and required less servicing. But even though there are still many around, they have not stood the test of time as well. If you want a square leaf shuttered MF camera system, it is one of the best deals out there now.
Some observations:
Keiv 88 lenses are noticeable inferior to Hasselblad, both in build and sharpness.
Pentacon Six Zeiss lenses are close to the quality (build ans sharpness) of the similar era Hasselblad lenses.
Mamiya 6 & 7, Bronica G1, and the Fujifilm GX680 lenses are excellent, arguably better than the Hasselblad versions. You really only see differences in rendering, which is a personal preference.