Yup, great camera. Beautiful image quality, decent metering, excellent ergonomics.
Everyone who really 'gets' the RF645 — that is, is satisfied with the available lenses, the frame size (6x4.5), and the portrait (vertical) orientation — seems to be very happy with it. It's kind of a zen camera: no fretting about the 'best' lens, or a vast array of accessories or upgrades. It is what it is. Just take it out and use it!
I have two bodies — the second purchase being the most efficient way to get a 45mm lens & external viewfinder, along with the polarizing filter kit. Though I'm glad I have a backup body now, I've still found the simplest expression of this camera is probably the best: single lens (45 or 65, as your preference), no external viewfinder, polarizer, etc., and set to aperture priority metering.
The only problem is the well-known issue of the mechanics of the winder. One of my bodies definitely feels looser in this area than the other. I try to wind it gently. Some folks have found that it helps to have the supply & take-up reels from the same manufacturer (that is, don't use a Kodak film and an Ilford take-up reel).
I've never figured out how to work the flash, though. Maybe I'm trying too hard.