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...
They really wound up a bunch of potentially compelling plot points over the summer. Maybe having those play out would have been a nice idea? I think it'll take a lot for me to stop watching this show, but I'm nervous about this season.