I regularly use a couple of different ones, one is a Red Rhino, not sure what brand the other one is offhand. Both are supposed to be 42' I believe - they'll haul 11 four foot bales, but about 6" of the last bale hangs off the end. They still sit firmly enough that they're not going to come out accidentally. We haul most of our hay about 16 miles. I can comfortably haul 12 bales on our 26' flatbed, but the extra time spent loading, strapping, unstrapping and unloading means I can haul 3 loads with the hay trailer in the time it takes to haul 2 with the flatbed.
About the only other thing we've used one for is hauling pipe, but I've seen them hauling any number of long things, like tin horns and roof trusses. But if you have much hay to move you won't regret owning one.