Concierge.com's insider take:

Edinburgh becomes an altogether different, wilder animal during the festival—well, festivals, to be precise. The edgy Fringe (comedy, contemporary music, theater, and dance), the highbrow Edinburgh International Festival (classical music, theatre, opera, and dance), and the International Book Festival, all run concurrently through August. The International Film Festival was moved to the month of June in 2008. The Fringe is the world's largest arts fest on its own and a full-on party from beginning to end, thanks in part to the 17,000 artists and performers who decamp here from all over the world. (The population doubles, so book accommodations as early as possible and expect inflated rates.) The Fringe is open to all "performers," so quality can range from the unwatchable (students from the back of beyond massacring Shakespeare) to the outstanding (Rowan Atkinson, Hugh Laurie, Emma Thompson, and Craig Ferguson all got their start here). There are also niche performances that you'll have to see to believe, like a retelling of Ovid's Metamorphoses performed in a swimming pool. Look to The Scotsman, London's Guardian, and Edinburgh arts and entertainment magazine The List to ensure that you don't book a dud. For more sober arts fans, the EIF is a full-on culture fix, with performances that might include all of Beethoven's string quartets over three days, Wagner's complete Ring Cycle, and an American Repertory Theatre production of Chekhov's Three Sisters. There's less fanfare around the book and film festivals, but they're not poor relations; year after year they turn out world-class programs. Start checking the festival websites in June and book early.