Been using Screenwriter for many years and v6 is a big jump ahead of the previous version (Screenwriter 2000). It's not without its frustrations but I find it a more robust and congenial tool than its only real rival, Final Draft. (Celtx and Montage are both coming along, but I'd not yet trust real live work to them. Yet...)
Screenwriter's strengths are, first of all, in its transparency. The only point of a screenwriting app is to get out of your way. Swapping fretting about formatting styles for fretting about your writing app is obviously no progress at all, and my experience is that Screenwriter is a bit less noticeable in use than Final Draft. Of course, Final Draft devotees may say the same thing about FD; it's maybe what you're used to that counts.
Screenwriter's foolishly-named "NaviDoc" is actually a very useful, very powerful outliner with total control over what is visible, what prints, and so on. Move the outline item and the script passages associated with it move too. The nearest comparison is the Mellel outliner. Screenwriter seems more robust, too. There's not that faint walking-on-eggshells I get with FD. Purely subjective but there it is.
Screenwriter is also much better at making sense of cut-and-paste or imported RTF files, which may or may not be of use to you. ***CAVEAT***: both FD and Screenwriter get completely confused if you import/paste anything using the SHOT element. Lord knows why. Final Draft 8 has the "FD Exchange format" which solves the problem; unfortunately Screenwriter can't export to that (though the brilliant Scrivener can).
On the downside, Screenwriter is less "Mac-like" than FD 8. Every time I fire it up (most days) I think: Lord, but this is fugly. But it's like an old Land Rover: may look crufty on the outside, but you forget about that once you're one the move and, instead, just enjoy its reliability. Screenwriter isn't as good as FD for reformatting -- going through an (imported, perhaps) script making sure all the elements are properly assigned. Creating a new element (I'm working with a LYRIC element a lot at the moment) is less intuitive with Screenwriter. And, like FD, it won't let you rename standard elements -- which can be a pain, frankly. Stage plays use "internal" and "external" direction, and it would be nice to be able to call your elements by those names instead of trying to remember that ACTION is now being used for INT DIRECTION.
FInal Draft is touted as the Hollywood "industry standard". Not really true, and shouldn't really be a major factor in your choice. PDF is the most usual way of transferring stuff, and if you get to the point where a producer/director wants a script in the other format to the app you have, there are plenty of ways around it.
For a standalone writing environment, then, Screenwriter, for me, wins easily. For a fine working combination, preliminary work in Scrivener and "final draft" in FInal Draft is probably hard to beat. I hope we'll get the same level of interoperability between Scrivener and Screenwriter one day.
In the meantime, it's worth remembering that obsessing about formatting minutiae is probably a displacement activity. (I'm as prone to it as the next guy. We're tool-using animals, after all.) The truth is, no screenplay which tells a good story and tells it well is ever going to be rejected because the Transitions are 5mm too far from the right margin...
In short, there's no screenwriting app which does everything. All of them have weaknesses; all of them have irritating "strengths" where they'll try their hardest to impose their will on the hapless writer. But for a robust heavy-lifting app with good production features and good links to scheduling apps etc., Screenwriter takes some beating.