I'll jump in. I have the 124G, an almost complete RB67 Pro SD kit, and an also almost complete C330 kit.
In my opinion, I think the Yashica 124G is way over valued, as a good one will cost in excess of $200. It's a very good camera; lightweight, excellent image quality, halfway decent meter, decent construction, but just over-priced.
The RB67 is priced nicely for the quality. Decent bodies can be had for about $100-150, and lenses can be stupid cheap. You can get the body and a lens for less then a 124G, and then have tons of room to grow.
The C series is also priced nicely. Again, it's a system camera, so there is room for growth. They are heavier then most TLR's, but they're built like tanks, like (almost) all of Mamiya's stuff. In the last year, lens prices seem to be on the rise, but they are still not ridiculous, and the quality is great.
If you like the 6x6 format, go with a C series, either a C220 or a C330, over the 124G. If that beautiful 6x7 negative is what you're after, and can handle the bulk and weight of the RB, jump on a Pro or Pro SD. I personally use my C330 10x more then my RB67, which I use 10x more then my 124G (I don't think I've picked up my 124G in almost 2 years).