Normally movies based around a character who is, well, an idiot, so to speak, are annoying and play up the main character as having zero idea as to what is going on around him. While Paul Rudd's character Ned Rochlin isn't the sharpest tool in the shed, he isn't a complete moron either. His character just wants to see the good in everyone, really, and that affects the way he perceives situations and people. For example, the movie is set into motion when he is arrested because he sells a police officer drugs since the officer claims he has had a tough week, and the job is getting to him.
When Ned is released early because of his good behavior, he heads back home where he is passed around by his sisters who want to help him but are hesitant to let him stay with them because he's that family member. As Ned travels between each of their homes, he uncovers secrets that each of them have and exposes them. Each family member must face their own problems and help Ned overcome his.
I do appreciate that Ned's character isn't completely stupid. You can see why he's so naive, and I think that's a major difference between a character like Paul Rudd's Ned and one like David Spade's Joe Dirt. Overall, I found the movie very enjoyable. If you like a good comedy, I suggest giving it a try.