Open or closed book exams?

Thanks for the responses to the last post, especially anonymous's. You gave me lots to think about, but I don't have time to respond tonight and have to go out of town tomorrow. But I promise I'll respond more fully. Again, thanks!

The short identification and passages sections of my exams are always closed books/notes. But I give my students a choice about whether they want to have open books/notes for the essay section. I think it's a wash, except for students who'll find having books and notes available comforting somehow.

Here's a little story: when I took my very first Shakespeare class, the professor said we could use our text(s) for the essay part of the exam. She also gave us three potential essay topics, and said that two of those would be on the final, and that we'd choose one for our essay. Being just a tad obsessive about school stuff by that time, I checked with her to make sure that it was okay to write notes in my books. She assured me it was.

So, I spent DAYS brainstorming about the essays, and finally wrote a thesis statement for each, along with an outline in the inside cover of my texts, and indexed the texts so that I'd have good examples at hand. (Did I mention I was a bit obsessive?)

And, of course, I barely cracked the books writing the exam (I think I used one once for a quotation), and did pretty darned well. I didn't do well because I had the text, though; I did well because I was incredibly well prepared to write all three essays. (Yes, I know, I could have prepared only two of the essays, but I wasn't sure which I'd want to write, and did I mention the obsessive thing?)

I pretty much always have students brainstorm possible essay questions during our review session, and adapt those to the real exam, so they should be able to prepare well. I don't limit their brainstorming to three questions, but do try to provide guidance during the review session so that they hit the big, broad issues we've been discussing during the term.

Here's the issue as I see it: Taking an open book/notes exam, someone who's unprepared stands a good chance of wasting a load of time looking for some information they want. Someone who's done an obsessively good job preparing should do well in either case.

The big question for me comes with those in the middle; someone who's fairly well prepared can mess themselves up by looking for some piece of information rather than paraphrasing and moving on. Or they can find the information quickly enough and look way smart.

My students always choose to have open books and open notes, though I do warn them about the searching and time issue things. And every semester, I inevitably see some student with my kind of indexing and such, who really wouldn't need the text or notes. It's the students in between I worry more about.

So, what do you folks see as the upsides or downsides to open book/note essay exams?