The little rinbergs, The Lovely Mrs. rinberg, and I saw Underdog last night. I enjoyed Insane Killer Bodyguard Brock Samson (Patrick Warburton) as the evil henchman, Cad, but if you are looking to entertain the adults in your party, you should probably go see something else.
Don't get me wrong, I didn't suffer like I have for other kids movies. It has some OK moments and I could enjoy it as long as I turned my brain off. It's just that it's definitely a kids' movie and shouldn't be analyzed closely at all.
Rotten Tomatoes has it at 19% right now, but I will point out that it did entertain the kids (ages 9, 7, 4, and 2), which made it worth the price of admission for me, since *I* didn't have to entertain them. Adults without kids may want to skip unless they're just big Underdog fans...
While I added it to my season pass list, I don't know how long it will be there. There just doesn't seem to be a whole lot of new ground being covered. Each of the main characters seems to be a rip off of a character in another prison movie.