It's mostly OEMs using BTX components, but some hobbiest builders do, as well. There are enough variations on the theme that one can build a very attractive small machine for nearly any need.
I am presently considering having several picoBTX systems built for a client. They will use significantly less real estate in the cubicles without sacrificing anything in terms of performance. The one thing the really small machines lack is a way to quietly cool them. They end up being among the noisier PCs because with so little internal air space, one has to blow the cooling air a bit harder than in a larger case.