We've got a 645Zi and a 645Ga. My brother got a Zi on our recommendation.
Autofocus, autoexposure, motor-wind, zoom, and even a pop-up flash! Think of them as the greatest compact camera ever built. They're so cheap, you really can't fault them. I'd agree you really don't want to be manually focusing them, and the manual exposure is a bit fiddly (as are some of the config options fidly to set), but either fully auto, or Av you can throw them around like no other MF cameras.
The individual cameras aren't a system as such, but the price is so cheap you can just buy another one with a different lens or format. We generally keep colour in one and bw in the other.
The Zi zoom is a bit limiting, but look on it as a bonus. It goes from slightly wide to not very long, but you wouldn't get a zoom lens for any other equivalent camera anywhere near the Zi price.
Obviously if you need Macro or some other specialist function, then look elsewhere, but for something to tote around they're great.
Ian