FD8 isn't so bad. It does its stuff and it's easier to set up new script elements than Screenwriter. ( I'm working on a musical at the moment, which needs a LYRIC element. In FD it's a snap. In Screenwriter you have to set up an "outline element" which isn't quite so well-behaved, though it does work.)
BUT... beware. The activation system will dump you like a shot if e.g. you have a system crash and reinstall, or (as I did) you upgrade to Snow Leopard then have to roll back because SnoLe isn't really ready to print with your new HP printer (tsk). The assumption is that all users are (a) in the ConUS, (b) have an internet connection and (c) aren't doing anything particular much that day in the way of work so can wait for a support dude to reset the damn registration database.. They can do what they like, but it's annoying for the user who, as I am, is on a little Greek island trying to get some work done in peace. Thankfully I'm only using FD to read/revise someone else's work. Screenwriter is a bit more elegant in its approach. Not much; but they've at least automated the process so you don't have to make two trips into town to get the software you've paid for working again.
I don't actually understand the reasoning, really. If you're a pro, you're going to buy the software because you need it. If you're not a pro, I can't see anyone popping $250 for a highly-specialised app, although they will certainly bootleg it to sit in their rooms pretending they're Bill Goldman. But I presume they have their reasons.
Oh, one heads-up. Lord Lightning mentioned integration with Scrivener. Which is wonderful if you're using standard screenplay format. On the current job, I'm not, and Scrivener (which is marvellous) and FD don't really understand what each other are doing. That pesky LYRIC element causes mutual incomprehension. I expect the brilliant Keith Blount will fix it in Scrivener but for the moment it's no go.