I use them and LOVE LOVE LOVE them. I've used every kind of fly spray I can find and also the BioSpot kind of treatment that is supposed to last a month or two. Nothing keeps my girl looking comfortable except her fly sheet. I still spray her legs, but that's it. So with the threat of West Nile so bad in my area, if I can add a layer of protection and help keep her comfy, then why not??
It also helps keep her cool. I was really concerned at first that she would be too hot in it. The first week she had it I would check on her a few times a day to make sure she wasnt sweating too bad under it. No matter what time of day, even on the 100 degree days, she was cool and dry under the sheet.
Sooo, less bother from flys, cooler in the heat, less sun bleaching. I certainly don't see any cons from using them.