They work well. I've ridden as much as 80 miles at a time with one, but I always replace it with a real spoke when I get home.
I am a bit confused. You have ridden 80 miles on one and you replace the spoke when you get home. Do you mean when you get to the end of the day or have you not used one in the middle of a multi-day tour.
My feeling is that since it takes maybe 15 minutes to replace a broken spoke with a real one I'd just as soon just do that right off the bat. Just one way to look at it though. I personally tend to think the fiberfix might involve almost as much fiddling around as just replacing the spoke with a real one (which is really a pretty straight forward operation).
BTW, I've found that while building wheels it helps greatly to have exactly the correct sized spoke, but on every one of the bikes I have toured on it was possible to carry one sized spare that would work for all three spoke types on the bike. They just need to be long enough to engage a few threads when replacing the longer spokes and not poke all the way to the tube when replacing the shorter ones.