As for background I have been a serious amateur for nearly 50 years, develop my own b&w and color film and prints, make 8x10 and 11x14 darkroom prints. 10 years ago I purchased an original manual focus P645 from KEH in EX+ condition and have the 45mm, 75mm, 120mm macro, and 200mm lenses and a P645 1.4x teleconverter, all also in EX+ condition. I shoot nearly everything hand-held. Film used is typically Kodak Ektar 100 and Tri-X or Fuji Acros 100.
In my opinion it is a superb system, reliable, and the lenses provide excellent image quality. Some find the inability to change film holders mid-roll, full stop exposure compensation, and the locations of the controls to be negatives but I don't. It's described reliability, excellent metering and mirror damping, and lens quality are what caused me to purchase it.
On the lenses, the 45mm, 75mm, and 120mm macro are all excellent. The 120 macro is special and I have taken many beautiful flower shots with it, again hand-held. I had read the 200mm lens is not considered as good as the other lenses but I have gotten very good results with it. I've found reduced sharpness using the teleconverter on the 200mm but results are still good.