I have two futons in the living room, so I keep a few different designs in sets of two. Every week or two, we change the covers & toss them in the wash. This works well for us since we have cats, and it's easier to wash futon covers than to vacuum couches. We've been doing this for several years, now, and all of the covers we've purchased were SIS covers.
For the most part, they hold up well. The material is slick enough to resist accumulating fur quickly. They also seem to resist stains somewhat (we're not easy on them). Although cat claws can definitely poke through, they don't tend to get them caught on these covers and they don't leave pulls in the fabric. The colors have not seemed to fade with all the washing, either.
Over this time, we've had 3 pairs (6 covers), and only one so far has had a broken zipper. We do try to be careful while zipping them, so as not to stress the zippers, but once on they're not stretched. It was disappointing to have to buy a replacement (replacing the zipper would have been too much work), but other than this we haven't had problems with them.