I sold a Hassleblad outfit to buy into the 6x7 in 1983 or '84 and have never looked back. The lenses were equal to or better than the Blads, especially the Pentax 55mm (I was doing a lot of interiors in those days). The variety of lenses represents all you could ever want and they're affordable. The 120 soft focus is without equal.
As mentioned by many above, you MUST have the mirror-lockup and always employ it at 1/30 second or slower.
Not mentioned is the fact that as long as the shutter is open, the battery is being drained. Time exposures really eat batteries quickly.
I have never trusted thru-the-lens metering so the metering prism is a moot issue for me, especially for interiors and night scenes.
They're also durable. Mine has been in terrible conditions (especially blowing dust/sand) and has travelled probably 200,000+ miles- and suffered the attendant vibration, shocks, and general physical insults. No problems-ever.
All that said, I also shoot a Mamiya 7, especially on the road when space and weight are important. It's lenses are minor miracles but they'll never have two lenses of the Pentax line that are priceless to me: the 120 soft focus and the 135 macro.