I have six Pentax lenses, and not one disappoints. But as has benn said, it depends on ones use and expectations.
The stars in my collection are the 35mm and 120 macro. The 75mm f/2.8 is also very good, even wide open.
Tax,
The 300mm is excellent but one needs top technique to avoid vibration.
The two zoom, 45-85mm and 80-160mm again should be used stopped down at least two stops, but then are as good as anything I've used.
I have used Hasselblad V and H lenses, Contax, and Phase /Mamiya. In real world shooting (not pixel peeping test charts) I find little that makes me wish I was using any of the former. But is I wanted some Hassy V series glass then a simple adaptor is all that's needed and CF lenses are available for a song.
Leica S lenses? Who wouldn't want them? But a proper set of glass will cost about the same as a mid-range BMW, and out of my spending comfort zone.
Cost is a real world issue. Pentax 645 has remained very popular in Japan while it mostly disappeared in North America and Europe during the past decade. If you look at e-Bay you see many lenses available from Japanese dealers in Mint or A++ condition for very reasonable prices. I bought six Pentax 645 lenses last month for just over $6000. Together with my 645z body, the whole system cost about $15,000.
So far the system totally satisfies from the perspective of usability, image quality and price. Putting something comparable together from Leica, Phase or Hasselblad would cost many times more.
My report is still a few weeks away, but in my view the Pentax 645z system currently trumps anything else. Perfect...no. Good enough for me? Yes.
Michael