I'll never say anything bad about Celtix...thats what I use and it's treated me well. I dont do very much and I'm FAR from professional, but it's free, helps you format properly, and has a bunch of cool features.
I'd have to bet it wouldn't hold up against "
real" screenwriting software, but like I say - free and simple. Thats enough for me.
http://celtx.com/