I recently bought the bright green ones for my horses and they work...eh. Sorry, I can't remember the brand, but it's not the one the OP is showing. Better than nothing, I think, but not nearly as well as fly spray. I just hate putting insecticide all over my animals and decided to give it a try. Flies seem to bother my horses' eyes the most, so I will probably just stick with masks.
The collars also aren't that durable, so don't expect them to last a long time.
Another downside is that their manes get all caught up in the velcro and creates a whole 'nuther problem with dreadlocks!
Oh, and everyone assumes they're cribbing collars! Lol!
I found them on clearance for about $5 each, and I think they have helped some, so I don't think it was a total waste of money, I guess. But in the future, I'll probably just stick to fly masks.