When I started learning screenwriting in the late 90s, copies of screenplays weren't easy to find. There was the bookstore (for mostly older scripts in that annoying book-sized format), there were movie memorabilia stores and conventions, and there was the Internet, but the online pickings were slim (especially since PDFs weren't widely used yet). In 2009, however, screenplays are all over the place. Where once I tried to read pretty much every script I could find (since I couldn't find that many), these days I have the luxury of being selective. I try to read scripts that interest me, of course, but I also want them to enrich my knowledge of both the craft of writing and the current spectrum of work being done by professionals.
That said, here's a list of all the different types of screenplays that are out there, in ascending order from least useful to most useful (in my opinion/experience).
1. Commercially available screenplays of already-released films. The biggest problem with many of these: they're not really screenplays. They're transcripts of the finished movies, written (arbitrarily) in screenplay format. They describe what was on the screen, not what was in a writer's head. Most other scripts of this type are at least shooting scripts, which, again, often have little to do with the work of the original screenwriter. Instead, they probably reflect months or years of doctoring by other writers; preparatory revisions by the director/producers; and overly specific instructions to various departments (camera, sound, etc.). Meanwhile, that great screenplay that one person (or team) actually sat down and wrote -- the one that led to the movie getting made in the first place -- is nowhere to be found.
2. Screenplays for movies I've seen (often coincides with #1). A screenplay needs to create a vivid, compelling movie in the reader's mind. If the movie is already in my mind, then I can't gauge how well the screenplay accomplishes that goal. The language of the script can only serve to trigger existing memories, which isn't the same as creating new ones. On the other hand, reading earlier drafts can be both interesting and useful, as it enables me to see what changes were made along the way. I've done this with the recent movies Whip It and An Education, and it's clear that a lot of adjustments were made to both prior to (or possibly during) filming. In the case of An Education, many scenes were re-ordered or cut out entirely, all for the better (although Nick Hornby's ever-impeccable dialogue remained untouched).
3. Screenplays for not-yet-released (or unproduced) movies that were written on assignment. These days, this is most of them; spec scripts rarely get bought and even more rarely get made. But assigned scripts are less useful to me, because I don't know all the behind-the-scenes. With some of them, the writer is simply given a title and premise and then sent off on her own; with others, the writer may have 50 pages of outlines and sketches and notes to which he absolutely must adhere. The bottom line is that there are bound to be a lot of decisions in that script that weren't up to the writer; and the flipside of that is that there's no way to know how that script would have been received as a spec. Still, they can be good lessons on the art of working within someone else's framework, which is pretty much what professional screenwriting is all about. And they can showcase some pretty great writing regardless of the circumstances.
4. Spec screenplays that either sold or got the writer an assignment. The Holy Grail. These are the scripts that took aspiring writers from the sidelines to the big game -- that forced some executive somewhere to open her checkbook and give a chance to someone new. They're the best examples of what Hollywood is looking for in new writers... and they're becoming harder and harder to find as the studio system contracts and 90% of all movies appear to be written by the same four or five guys. Nonetheless, they're out there, and many of them are pretty thrilling reads from exciting new voices. These people made it; now it's up to me to work my ass off enough to follow in their footsteps.