I felt Siskel and Ebert really missed the boat on this one
Typically I will agree with one or both of their opinions. But here I think they were really unfair slamming Sister Act. Siskel even said that the people in the audience he saw it with were laughing and enjoying themselves. Yet he said it was too predictable. But what did he want? The Usual Suspects? Does every movie have to be totally unpredictable? Come on. Even though the film was a little predictable, I don't think it detracted from the humor.
And Ebert said that the director had no style and the trailers spoiled most of the jokes. The director, Emilio Ardolino, was making Sister Act not Lawrence of Arabia or The Godfather. How much style do you want from a simple comedy? And I bet if Ardolino got all stylish with the direction, they would've complained about that. As for the trailers, it's not the film's fault that they spoiled the funny scenes.
I watched it again recently and found it to still be pretty funny. The film was just meant to be an enjoyable crowd-pleaser, and I think they ought to have enjoyed it on that level and not had such unfairly lofty expectations for a comedy. Then again, Siskel & Ebert gave the abominable Speed 2 two thumbs up, so their opinions should definitely be taken with a grain of salt.