Although The Second City originated in Chicago, I must say that the Mainstage is one of Toronto's comedic treasures.
Shows are well-written and performances rarely disappoint, so people in Toronto (whether a visitor or resident) would find it a good idea to put it in their downtown evening itinerary for good laughs.
What you would expect to see on an average night is basically sketch comedy theater with an improv performance after (enjoyable), with non-noteworthy food and sometimes cramped seating (unenjoyable). Thankfully, the show's content is its saving grace.
My pro tip: If you're staying in a hotel, get a hold of those discount cards. If not, there are three hotels near The Second City Mainstage (Hotel Le Germain, Hilton Garden Inn, and Hyatt Regency) and I suggest you check their respective lobbies to see if those discount cards are there for the taking.