I've written a screenplay or two in my life. When the iPad was released more than a year ago, I imagined that it could, saving a trip to a printer or laptop. Well, so far, the iPad's been great for a lot of documents and publications...but a little slow on the uptake when it comes to the complicated formats of screenplays.
For my last screenplay revision, I actually saved my script to PDF format using Final Draft, then opened upto check out my work. iBooks has its own PDF reader, too, but GoodReader adds a surprisingly robust set of annotations for marking up your draft.
For those who require native readability of screenplay formats, Celtx Script and Scripts Pro currently support some basic writing and editing functions, but have their ups and downs. Celtx works with its own software, whereas, but doesn't have the best script viewer. Final Draft is supposed to be making its own app, but it's been indefinitely delayed.
Until Final Draft's official app debuts, there's the new-on-the-scene FDX Reader, which reads pesky Final Draft 8-created files (.fdx) natively, saving the extra step of having to make a PDF version. It also works with Dropbox, or imports via an e-mailed attachment or iTunes. The app is minimalist: the parchmentlike paper background is softer than a white PDF, and text sizes can be adjusted unlike a PDF. Pages turn more like they do on Amazon's Kindle app than the way they do on iBooks. FDX Reader also works in landscape mode, but doesn't offer two-page viewing, instead offering an odd full-page landscape view.
FDX Reader ($8), created by screenwriter John August (his screenplay for Big Fish is included for free with the app purchase) will break down the script with dotted lines to indicate "actual" script pages, and includes scene numbers, but doesn't show revision markings. It also can't be used for edits, revisions, or annotations. That's pretty limited stuff for an $8 app, but producers or script readers who possibly get hundreds of scripts a month could find this reader pretty helpful. It's clearly priced for Hollywood.
For the rest of us, GoodReader (currently $5) is a more versatile option. And, of course, iBooks offers plain PDF reading for free, and that's not bad, either.
For comparison purposes, take a look at screenshots of both apps. Honestly, PDF viewers will do the trick for most of us until a true Final Draft editing/annotating program becomes available. GoodReader's markup capabilities stay glued to the PDF, but they effectively work as well as pen marks on a printout: they're useful note-taking tools in a pinch. But, if you're buried in FDX files somewhere in Studio City, give FDX Reader a whirl--or wait for a sale.