Whilst obviously an NIC onboard or on a card is preferable.
Which ones work well in practice with the Linux kernel?
How much trouble in practice is having the cable connector sticking out the back (front?) of the machine?
There’s some neat small factor low watts designs about that would make great SOHO servers, if a 2nd NIC via a USB port was a reliable solution.
The good news is that nowadays, most NICs are supported quite well. The only exception might be a very old or very new card for obvious reasons: the older one may have been dropped and the newer one may not have made it in yet.
But since you’re looking for actual experiences, I’ll share mine. We have a LOT of machines at work, so we’ve run across most common NICs at one time or another. The only real answer is to do a Google search on the model you’re interested in (use “+” and “-” and put the model in quotes to force Google to look for that exact string: ex.,
+linux +Broadcom +“bcm5708”
As long as you physically protect the machine from stompy-feet, you can put the adapter anywhere. 
Have you actually used USB adapters? Sorry I now see that I should have re-used the title, in the body of text, to make it clearer that I’m trying to find out more about USB Ethernet, not NICs in general which use PCI or similar.
The bcm5708 looks like a PCI Gigabit Ethernet. Rather than PCI card, I’m asking about cheap USB 2.0 to Fastethernet RJ45 adapters.
One issue is, that product details about actual chip can be unclear from packaging, there’s lots of “Windows” stickers on them though.
I’d expect them to be recognised, but how well do they work in practice?
Are they stable enough to use for the Internet Gateway in a SOHO environment?
I did miss that at first (whence the little disclaimer that I had edited). But I think you missed my suggestion: do a Google search on the make/model that you’re interested in. See what people everywhere are saying.
That said, I hope that people here who HAVE used them will post here. I’d like to know myself.
Are they stable enough? I don’t see why they wouldn’t be, but maybe someone else here could pipe in. I’ll tell you this: WE don’t use them, for a couple of reasons. First, because of the lack of track record. Second, because of fears about speed issues. Too many USB devices (and/or drivers) will drop back to the equivalent of a 1.0 specification if they run into anything that they don’t understand.
Now, in this case, if you’re using it on the Internet side of things, that won’t matter … unless your Internet service comes via a T3 bonded to a fiber backbone. 
(Oh, how I wish, and oh, how jealous of you I would be if you DID!)
And just for completeness, we’ve been converting over to managed switches instead of “whatever’s on sale this week,” and it has been an eyeopener. All network devices – USB, wireless or fixed – apparently want to fall back to much slower speeds, or even to half-duplex, if they run into any problems. I guess the manufacturer figures that’ll cut the support calls: most people won’t notice, and at least it works, right?
Looked in a place, and found the packaging very uninformative, cheap commodity items.
Have seen them “slagged” by ppl who didn’t seem to have used them, but it looked like FUD. If ppl are not using them, then I’m not going to be guinea pig, on a main Internet 10 Mbs connection, which makes USB 1.0 speed a likely bottleneck.
That said, I hope that people here who HAVE used them will post here. I’d like to know myself.
Are they stable enough? I don’t see why they wouldn’t be, but maybe someone else here could pipe in. I’ll tell you this: WE don’t use them, for a couple of reasons. First, because of the lack of track record. Second, because of fears about speed issues.
Fears is the reason, why I thought I’d ask a question, rather than try and answer them for a change
I have more than 600 machines running openSUSE and all of them on network without any problems. Also, I have used USB Ethernet adapters for the last couple of years in a production environment with no problems reported.
All machines are of different brands and many different network cards.
What models/brands? Do you remember off the top of your head? That would be useful info. And can you confirm that they’re operating at full speed?
Remember, my other concern is whether the USB NIC is actually running at full blast. I’ve had problems with other USB devices silently dropping back to 1.0 speeds without even telling me. That’s not a big deal on a typical DSL-based Internet gateway, or even on a typical network where people are just printing and browsing.
But in our case, we have audio workstations all over the building that MUST communicate at high speeds. We’re editing, moving and transporting real-time audio for 5 radio stations. Can’t afford bottlenecks there … 
That’s nice to know. Did you go with branded USB Adapters or just pick up cheap Generic ones?
I’ll presume you’ld notice if you weren’t getting Fast Ethernet 100 Mb/s speed from them.
Network cards:
We have a mix of 10/100 ones and a lot of them came on unbranded machines. We also have branded machines like HP, IBM, Dell, Sun etc. and they came with built-in network cards. We had problems with one Sun machine whose network card used to hang after few days. However, later deliveries of the same model do not have this problem.
USB-Ethernet:
I have to check the speed. It is in a protected data center and I can check it when I get a chance to go there again. However, it was just picked up from the local market (Dubai). Some Taiwanees ones.