Like some of the others here, I like the 6x6 square format, too, and I like the compact, self-enclosed nature of TLRs, so I use a Rolleiflex most of the time. I probably alternate about 50/50 between MF [almost all with the Rollei, although I do own other MF cameras], and 35mm, where I use rangefinders and compacts most of the time, with the occasional SLR.

I find a TLR suits my way of working; it's compact, the image quality is more than adequate for me, it fits in a small bag and isn't any larger or less convenient than a 35mm SLR. I also like waist-level finders, so that suits me, too. I agree with everything jp498 says about the friendly nature of the TLR when shooting people.

I've experimented a bit recently with 5"x4", but, so far, I haven't found it offering me much that I can't get from my Rollei in terms of image quality [I'm not set up to make big enlargements].