I have a GA645Pro, a forerunner to the 645zi. If the new camera is anything like the GA645Pro, then it'll take great images but it's the closest thing you'll find to a medium format point-and-shoot. (The biggest difference between the two cameras is the lens. The 645zi has a 55-90mm zoom lens, whereas the 645Pro has a fixed 60mm lens.)
You can manually focus the 645Pro, but you won't want to. Tap the AF button, then hold the MF button and spin the wheel...and watch the numbers change in the viewfinder to indicate the distance at which you're focused. There's no rangefinder patch in the viewfinder, but thankfully it is parallax-corrected.
Manual exposure is pretty easy, and the built in meter is good. Aperture priority and Program mode are both supported. The lens on the 645Pro is top notch, and the images are excellent. But the camera really is made for automated operation, and focusing it manually feels more like a work-around than an elegant solution. I don't know if the zi improved on this or not.
Having said all of this, I'd buy the camera again in a heartbeat if I didn't already own it. Having a reasonably sized, quick-to-use 6x4.5 camera is a very nice option.
Good luck with your decision.