Not normally one to bang the drum about proprietary software - someone usually comes along and says you can make your own from tin-foil and and rubber-bands for half the price - BUT...
As it so happens I'm using it right now and a promo popped up on my Facebook wall which reminded how useful it's been.
http://www.screenplay.com/p-77-outline-4d.aspx
Not on a percentage or anything but if you're into longer format stuff - comedy/drama, sit-coms with a series arc and of course fillums it's worth taking a look.
It's not one of those packages that tells you how to write either - which are infuriating at best.
Money well-spent IMHO.
Anyone else have any experience of it?