The first lens I owned was a Promaster 28-80 that was a real piece of poo. The second lens I got was the 100-300. The first copy I got would give an Err99 error whenever I focused in a certain way. I took it back. Eventually I found another copy that didn't give the error and I bought it cheap.
Compared to the Promaster, it had pristine optics. It focused quick and was quiet. It suffers from zoom creep pretty bad if you walk around with it. Since I bought it, I've developed a better eye for what differentiates a good and bad lens and I'm not quite as impressed with it. More importantly, I realized that I don't really want a zoom like that.
If it you can get it cheap and it isn't a mission-critical lens, it can be fun to play around with.