Saw a production of The Breakfast Club by D2 Productions who has been staging shows there for some time now (check out their shows when you can) .
Located in the heart of the plateau, it is no surprise Mainline is also at the heart of the Fringe Festival. It houses two theatres (Mini Main, Mainline Theatre), a nice touch is the bar in the lobby. Ya, thats right, you can drink you favourite libation while watching a production of the Bacchae.... Ugh, bad joke.
Support local theatre and check the place out. Not nearly as stuffy as the Segal Centre or as prententious as the Centaur.