Screenplay format is crucial. I have no idea how the hell people in the old Hollywood days wrote screenplays with typewriters. I guess you get used to what you get used to; like cold weather. After a week of using Final Draft Software I have to say it operates a little more smoothly than other software I've used. I haven't had much of a chance to try out many of its features, but, I have found it a little more intuitive.
In the past several years I have used Celtx (free), Five Sprockets (almost useless web-based software), Movie Magic (great stuff!), and one more piece of freeware the name of which escapes me. You know, tonight at 2 a.m. I'll remember. It was something like Cinewriter? Honestly it worked pretty well. You can also use templates in MS Word or a similar word processing program if you are savvy enough. I'm not :) Google docs also has a template out there but I couldn't really get it to work. Google docs have their place, especially if you want to collaborate with someone. I think a lot of the free and commerical software (including Final Draft) also allow collaboration. My biggest gripe with Celtx was with the pdf rendering.
I'm curious about what my readers use and why. I chose FD because my understanding is that it is becoming the industry standard. Interestingly there is a great wikipedia page out there http://en.wikipedia.org/wiki/List_of_screenwriting_software that lists a lot of the programs and web utilities out there.
I'll shut up now.