I don't use one, but there are a tremendous amount of Smoothstepper users at CNC Zone. And most everyone who uses seems to love it.
Yes, I believe it's an FTDI USB chip, as the FTDI driver is on the Smoothstepper site.

The general consensus seems to be that the ethernet SS is the only way to go these days. The USB version has always suffered from noise issues. Some have no problems with it, but a lot of people do. The ESS is said to be pretty much immune to noise.