I respectfully disagree. 645 is far more flexible than larger medium formats. Cameras are generally lighter, richer in features and faster to use, systems are usually more comprehensive than and the DOF issues that plague 6x7 or larger are not as pronounced.
Originally Posted by Goldfarb
To me a 4x5 and a 645 are perfect companions. When the former is impractical, the latter will often get the shot. 6x6 is not everyone's cup of tea (and you usually end up with a 645'ish crop anyway), 6x7 and larger get very bulky and suffer greatly when extreme DOF is needed.
If you're willing to deal with the bulk of 6x7, may as well go straight to 4x5. If you want flexibility but can't live with puny 35mm, 645 is the logical choice (and will still get you pretty nice 16x20s). Both together will cover you for most situations.