I'm a Pentax 35mm user. While I haven't used the Pentax 6x4.5 cameras, I understand they use the metering and focusing technology from their 35mm line. My 35mm equipment focuses really fast! The only downfall is the loud focusing motor.

The 645NII has only one big problem as far as I can tell - I don't think there is a waist-level finder option. On the good side, it has multi-segment, center-weighted, and spot metering, 2 frame/sec motor drive, predictive auto-focus, and mirror lockup. I think it would be worth investigating further.