It's probably not very far-fetched to think of Raju Hirani with a wand, infusing his own peculiar brand of humor, cheer and charm into these stories where another director might have probably gone a different route (Farhan Akhtar and Shimit Amin come to mind, for instance). Although "3 Idiots" is not quite as charming as the "Munna-bhai" series, it is neverthless very enjoyable fare.
Aamir Khan acquits himself remarkably well as the college-goer (notwithstanding the rumors of air-brushing the wrinkles). Although he went a little overboard with the shuffling body language initially, it soon wore off and Aamir soon got into character as the "different" Rancho. However, this is probably the zillionth time Aamir has undertaken this character graph - the I-don't-care brat who slowly comes to terms with where he has gone wrong and straightens up (Jo Jeeta Wahi Sikandar, Ghulam, Dil Chahta Hai, Akele Hum Akele Tum etc). Neverthless, he never gives us the "seen-it-before" feel, and summons up a warm camaraderie with the rest of the college-going crowd.
Madhavan is as dependable as ever. He gets some of the best one-liners in the movie (the one after the child-birth scene is priceless). Sharman Joshi, funnily enough, does not get to flex his comical skills much (I thought he was unbeliebably funny in RDB). However, he shone in the "interview sequence". The actor who played Chatur was the best of the lot - he was just spot on. Kareena had nothing much to do. Boman Irani was, quite frankly, a major disappointment - this is a performance unbecoming of his talent.
In the end, this is completely a director's movie (unlike a "Ghajini", where it was the stars who moved the movie forward). There were some superb directorial flourishes - the whole sequence with Sharman's parents was very well conceived; without resorting to the typical cliche, the point was made, and with humor too. Chatur's "balatkaar" speech, the re-union scene, and the many one-liners scattered throughout the movie ensured that there was barely a dull moment.
However, there were quite a few "boring" sessions as well - most of the songs were not required. Sharman awakening from the coma took quite a bit of screen time - this is a flashback, and we have already seen Sharman hale 'n' hearty, for chrissakes! The birth scene, while a nice high-point in the flow of the story that finally showcases Rancho's inventiveness, went on for too long - and the "all is best" motto was downright stupid.
However, all said, this was leagues above the regular stuff that Bollywood turns out. - extremely enjoyable movie.