I quite like the 645 format, it's a significant jump from 35mm to give a very significant increase in quality and still be portable. I've had a couple of M645's for over 30 years with 80mm and 45mm lenses. The 670 is only a slight increase in format size over a 645.
Having said that the MF cameras I tend to use the most are TLRs, and I've just been shooting with my Yashicamat 124G in Cyprus, I use Rolleiflex cameras here in the UK. I like the square format but I'm usually using a TLR alongside a Large format camera (5x4).
Ian