I used Celtx for years but recently abandoned it because it's becoming too bloated. I use FD 8 and Trelby (free, runs on windows and linux, plays well with FD)
i have fd8, and as someone who never wrote a script before until this year, i found it dead easy to use and without it id be dead... over exageration but worth it
However, I'd rather direct a skillfully composed story written in crayon on napkins than some mediocre drivel written in the most advanced and comprehensive software on the market, so I'm not sure why it matters.