I've used them for the last 3 years.  Neighbors claim the fly population has decreased a lot.

I still see flies, horses still stand and swish their tails. So I don't know. 

But if it keeps my neighbors from complaining to the city, it's worth the $100 a year.

A couple key facts.

The predators have no effect on adult flies.

Fly predators only attack fly eggs laid near where you release them. They have no effect on adult flies that travel from someplace else to your yard. So if there is a feed lot of other source of flies 1/2 mile from your home. You will still have flies.

Predators work well as an addition to a well managed fly program.  Clean manure management. Fly traps, etc.

I am starting my third year with Arbico Fly Predators.  We are in the woods so biting flies are extreme from mid July - September...I'm talking about flies that bite right through heavy denim and as you would expect, drive the horses insane with their painful bites. 

 I believe the flies have been reduced but by no means eliminated.  I will keep using them regardless, as any reduction is appreciated.  And I only spend about $20/month April thru October.  Not bad considering I can pay the same amount for a top shelf fly spray product and it would only repel the flies for 15 minutes!  My solution, horses inside a dark barn all day, ride in the evening or very early morning, and buy lots of Skin So Soft - which happens to deter the flies the longest.  Too bad if makes such a mess of my horses and tack though!   Some times it makes me want to move, but the spring and fall weather do make up for it!

My last youth client bought some for her home barn, they all moved away or died.  I've never tried them but I don't have fly problems because of the way my barn is built and cleaned regularly.  (My barn is cleaner than most restaurants.  It smells faintly of pine-sol and bleach, not manure and urine.)

The neighbors barn is another world completely and when the wind blows you can tell.  No ventilation, no cleaning except once a year and this only covers the floors.  I don't touch things if I have to go inside this barn, I know I'd die from whatever I caught.

I may have turned the horses into "prissies."