I'm old enough to remember going to seven Edinburgh Fringe shows in one day at 50p per show. Some were rubbish but who cared at 50p a ticket. Nowadays, choosing a show is a more serious activity with tickets costing an average of £7.
Never fear, checkout www.festivalpreviews.com. This website hosts video preview clips of the shows so that you can see for yourself whether you might like it. The site also has festival clips from Brighton, Adelaide, Avignon, Tokyo, etc etc.
I strongly recommend a little extra time when planning your transport time in the summer months. Most Swedes take all 5-6 weeks of vacation in one stint in July or August so buses and train schedules are greatly adjusted for what staff there are.
www.sl.se for the schedule and www.xpats.nu for other advice from the discussion forums frequented by English-speaking locals.