I've used the fishing line and so far, so good. We have Cooper's Hawks here all the time, and before I put up the line one of them killed a NH pullet. Since then, no losses. We also have a good rooster, but so far I don't think he's had to do more than scream in warning.
Fishing line is super-cheap. Get 15 or 20 lb test, I think it's like $2 for 300 yards. I put in a few extra-tall T-posts (the 10' kind, so they stick up about 8' + ) and ran a line down the center of the run, then ran zigzags over that line, so I can walk without garrotting myself. My yard is pretty big so there are large gaps, easily enough room for hawks to fly through, but I think the appearance of the line throws them off. Cooper's Hawks come in low, eye-level or even lower, and then swoop over the fence to attack. The webbing of fishing line at eye-level apparently looks impenetrable, or at least not something they want to tangle with.
Ravens are another matter... they will drop straight down through the gaps. I have to solve that problem before I can let my new chicks out.