The folks at Muddy Hill Farm in Montgomery County, Maryland grow blueberries and raise chickens and pigs. Hence, they have flies.
They recently put two fly traps to the test -- the RESCUE! Disposable Fly Trap and the Black Flag Flying Insect Trap -- and posted the results on their blog.

They posted a comparison of the two, along with results on Day 3 of the test, in this blog post.
And the results after Day 7 of the fly trap test are posted here.
Bottom line: It looks like RESCUE! outperformed Black Flag by three times, at just one-third of the cost.

Ant Baits
Birdseed Moth Trap
Fly Trap Max
Fly Trap, Big Bag
Fly Trap, Disposable
Fly Trap, Fruit Fly
Fly Trap, POP! Fly
Fly Trap, Reusable
FlyPad
Japanese & Oriental Beetle Trap
Spider Trap
TrapStik, Carpenter Bee
TrapStik, Deck & Patio Fly
TrapStik, Indoor Fly
TrapStik, Wasp
W·H·Y Trap for Wasps, Hornets & Yellowjackets
Yellowjacket Trap, Disposable
Yellowjacket Trap, Reusable
Ants
Biting Flies
Carpenter Bees
Flies
Fruit Flies
Hornets
Japanese Beetles
Mud Daubers
Oriental Beetles
Birdseed & Pantry Moths
Spiders
Wasps
Yellowjackets