The episode took off once Mysterion appeared. Before it was just another Cartman scam episode. It actually reminded me of the ATHF ep, "The Drizzle". But Mysterion turned out to be a great character; he looked out for real crime and he could do the cool superhero bit down pat.
I thought the ending was well played. Mysterion did reveal his face, in a clever satire about how SP faces all look alike. Let's see, Mr. Garrison said he was one of his students, Cartman said he called him out before, and it's obviously not Timmy, Jimmy, or Token. That previous scene indicated that he's not Kyle (though what if they switched costumes after that scene?...) I'm guessing that it's Clyde, followed by Kenny, followed by Stan, and just maybe Wendy.
So why didn't they demand the Coon unmask as well?