First of all, I'd like to say that I really enjoyed this movie. However, that said, I can't say that it was a "good" movie. I went into the theatre with pretty low expectations (something I've learned to do). I'm glad I did, because the plot and development wasn't stellar. The jokes were low-grade humor. But, I was in the right mindset to enjoy them.
I wasn't the only one. In fact, the entire theatre was laughing out loud. I didn't hear anyone complain after the movie came to a close. I even saw one guy fall out of his chair laughing! But again, I have to warn you, Grandma's Boy is NOT a top-quality film. It is a funny (albeit low-brow) movie, and if you go with the right mindset, you'll really enjoy it. Oh, and don't take kids to it.