I have one on my '90 and I like it. The longer I have it, the more I think this style really does work.
It does protect from rocks (tho I got a small stone stuck under it and had to pry it out), you just have to be sure to clean it regularly. It may still make your paint in that spot look different after it has been on for a while. There are products out there that just stick on and give similar protection, clear vinyl sheets and such. I like the looks of my deflector tho.
As far as driving behind semis and dump trucks: I was riding with a fridn when a grapefruit sized rock came out from between the duals on a dump truck...smashed through the windsheild! Pretty nasty! Glad it was not my rig!