Great camera. I see that one person mentioned the 120mm f/4 macro. It's wonderful! Sharp as a tack at f/4 and allows you to cover a range of shots but for portraits it's simply amazing. It's a bit more expensive then others, I got mine on ebay, a 9+) for $210 while KEH has the same for over $600.
What I'm so happy about the 645 from mamiya is that the Lenses are so inexpensive relative to their 35mm counterparts but the quality is just along there with them. Amazing values to be had.
Do yourself a favor and check out the 120mm Macro and if it fits, grab one.