I started with a Mamiya C300f TLR as well. Very good place to start. IMO, 645 is a useless format. It's just not enough bigger than 35mm to justify the price increase. 6x6 is great if you're printing square or nearly square. It is, without a doubt, my favorite format. However, if you're cropping 6x6 down to make prints people have frames for (like a typical 4x6" frame), you've basically cropped it back to 645 and we're right back at "not big enough to justify the cost". I have a ever-growing Mamiya RZ67 Pro II setup basically because it'll do damned near everything (even though other cameras might do specific things better, the RZ will do them ALL) and the 6x7 format is nearly crop-less to most normal sized enlargements (5x7, 8x10, 11x14, etc) so you get to use (nearly) 100% of the big film you're lugging around all that metal and glass for.