I've never used one or seen one used, although I have seen them for sale, so I can't comment from my own experience.
I would suspect it'll work fine, just not as fast as a "normal" NIC - well, of course, since USB2 is 480Mb/s and Gigabit Ethernet is, well, 1000Mb/s, heh. But leaving that aside, I think you might see slightly higher CPU usage but nothing major, I would imagine it'll work fine for general internet usage and normal home network usage, just don't expect to see "native" performance for huge file transfers.