I've bought a couple of USB gigabit ethernet adaptors now, one aimed at USB 3.0 and one aimed at 2.0. This turns out to be rather good.
USB 2.0 runs at up to 480 mbit (although bus access constraints limit that to 280 mbit), so it should be helpful for an ethernet that feeds at 1000 mbit, although not as good as a PCI card. That's just as well, because most of my boxes run USB 2.0.
These adaptors are useful for boxes that don't have a spare PCI slot (at the moment, that's only one box, the one called "Heavy") and for boxes that I want to temporarily boost from 100 mbit, but I don't want to mess around opening up the box and adding a card.
But I have a plan for Heavy. Remember I ordered a SATA 1 to 5 port converter? I'll put that in Heavy, and that will free up a PCI slot; then I can put a gigabit ethernet card in.
That will also be a good test of these converters; if it works well, I'll order a few.
I've tested the USB - ethernet dongles with Fedora 17 and upwards, and the operating system detects them and uses them without me having to do much.