The most impressive thing about 'My Idiot Brother' is that it accomplishes something few films deem important these days: It makes you want to be a better person. So, it's hard to fault the film, which carries with it a few speed bumps and an ending that feels a bit too cookie-cutter, because it just plain makes you feel good. From the story to the performances to the many unexpected moments of hilarity and heartache, there's a reason why 'My Idiot Brother' is one of the biggest (and most well-received) films at this year's Sundance Film Festival, and it begins (and ends) with one of Paul Rudd's greatest roles.

Rudd plays Ned, a hippie with a heart of gold who lives out on a farm with his hippie girlfriend, growing produce (and marijuana) while enjoying a quiet life full of farmers' markets and downtime hanging with his dog, Willie Nelson. But Ned isn't exactly playing with a full deck, and when he sells pot to a uniformed police officer, Ned is arrested, thrown in jail and replaced on the farm by another hippie boyfriend who's just as nice and honest as Ned. This little inconvenience forces Ned to leave the farm and shack up with each of his three New York City–based sisters (Elizabeth Banks, Emily Mortimer and Zooey Deschanel), who soon realize that their idiot brother is the best and worst thing to ever happen to them.

This would probably be a completely different movie if it weren't for its superlative cast, which stands as one the most successful ensembles at this year's festival. Not only are Banks, Mortimer and Deschanel all hilariously complicated in their own unique ways, but some of the film's supporting actors, like Steve Coogan, Shirley Knight and Adam Scott, manage to steal virtually every scene they're in.

'My Idiot Brother' is truly is Rudd's show, though, as Ned bumbles from one trendy New York apartment to the next, unwillingly tiptoeing across each sister's fragile personal life. Naturally Ned somehow finds a way to accidentally screw something up for each sibling -- from ruining a marriage for one to exposing infidelity for another -- but the funniest part about it all is that Ned is clueless the entire time. He's the live-action version of a dog -- of man's best friend: He loves everyone unconditionally, and expects those who surround him to be good-natured, down-to-earth folks just like him.

Except it's the complete opposite. Everyone around Ned is angry, detached and self-absorbed. To them, Ned is a slacker who has no idea how to manage his own life. He's pathetic, needy and a constant thorn in everyone's side. You can understand how that sort of tone could eventually crack through Ned's idealist persona, and you'd be right in predicting that it's only a matter of time before he snaps.

This is director Jesse Peretz's ('The Ex,' 'The Chateau') best film to date by far, though he finds a good amount of help in the script from first-time writers Evgenia Peretz and David Schisgall. Their screenplay does a fine job of mixing familiar Hollywood ideas with an indie flavor, and though the flick has a bit of a rough go while tying things up without feeling forced and cliché, 'My Idiot Brother' should still end up one of this year's most audience-friendly comedies. And that's not such a bad thing.