I also use the F1, not the SX, but they are very similar. The F1 is just a smaller SX, the SX has a few more routing options and more channels. I bought my F1 new and I used it as a studio and live mixer for about 3 years. After that I only used the F1 for live, and sometimes as a sub mixer. It's great for live because it easily fits in a suitcase with some other gear, unlike the Mackies that you often see on stage which are too chuncky. And it is very reliable, it has been traveling with me for almost 10 years now and it still works very good (much better than it looks ).
Soundwise, it has always been a very quiet mixer. The EQ is good, perhaps a bit boring but still musical enough to record some decent tracks. Only thing that sucks for live is that the headphone out mutes the monitor out. I've had a few soundguys getting grumpy when I told them that I don't want to use the monitor out from my mixer.
As for the build, I really think it's extremely reliable. As said, mine has been traveling for 10 years and to this day I have hardly any problems with it. It has taken quite some beatings and it really looks worn with it's bend faders and bend pots. But the only thing that is showing signs of wear is the headphone pot meter; a few months ago it started to crackle. One thing to be aware of is the psu input. Mine felt wiggly since day day 1 and I was afraid it was the first thing to die. I'm amazed that it actually didn't die in all those years, but I have seen a few SX's with psu input problems so it is something to look at.
All in all, I can't really think up anything against these late 90's Folio mixers. They are low budget and a bit sparse, but they are reliable and they sound decent. If mine dies tomorrow, I'll get the same one the day after tomorrow, no doubt.