Someone suggested this to me, and I've been thinking seriously about purchasing a copy, following some positive reviews I've read at various sites online. What my question is is whether or not it provides to the playwright anything above and beyond the heap of opportunities and calls for scripts that our diligent moderators search and post on this board? I did read that it lists contact information for close to every regional theater across the country, which could be a great help. Just fishin' for a few more opinions before I make a decision.
If you're a member of the Dramatist Guild then you already get a great submission guidebook. If you're not a member of the DG then you should purchase the Sourcebook. It covers additional opportunities to the ones posted here, as well as information about colonies, fellowships, agents and additional resources. It is a more comprehensive resource than this site alone, or other internet resources alone.
Most of the internet sites tend to post announcements for calls for plays -- especially contests.
It does not, however, post extensive submission requirements for theaters (unless they have a contest or an annual call for submissions.) Without it you'd be missing out on many opportunities -- and a way to familiarize yourself with the submission requirements of many, many theaters across the USA.
Also -- part of what I do on PlaywrightZoo is host a resource that acts as a companion to the sourcebooks -- by having extensive links to theaters that either accept full plays, or theaters that accept queries. I'm still working on the second set of links (I'm on S) -- I work on it when I'm on break from school. I should have it finished up by this summer.