Pentax 67...go try one. If you want an easy transition from 35mm, this is for you. The lenses are first rate and the camera is a tank. There are many possible configurations and an extensive array of lenses. That said, you should expect to do almost all you shooting with a tripod...the camera is relatively heavy (though the lenses are not) and the focal plane shutter and mirror slap can mess things up way too easily without one. If you're used to shooting a mess 'o film handheld and on the fly stop reading right here!!
I chose to avoid any of the metered prisms in favor of a spot meter and modified zone approach, but they're certainly worth considering. Since the Pentax 67 II has now replaced the Pentax 67 (an apparently long awaited update), P67's are probably much more reasonable in price. When I bought mine (new) a camera, prism finder and 3 lenses (55, 105, and 200) was one third the price of a new hassie and comparable set of lenses. It's also worth noting that the 6x7 negative is a lot bigger than the 6x6 or 6x4.5. I'm now shooting as much 4x5 as I can and not finding the quality noticably different (though the 4x5 has other wonderful virtues).
Having said all that...sheesh, I can't believe I'm writing this...what Sean says is also true. Good luck!!!!