Karan (Saif) is a cartoonist about to go to America to attend university. At the airport, he passes Rhea (Rani) being reluctantly released to do the same by her doting mother "Bobby" (Kirron-ji). Before he departs, he manages to take a few cheap shots at women (in front of his mom--Rati's character--no less) and prove his general forgetfulness. Good thing he has a friend like Mihir (Jimmy) to remember all the things he forgets.
On the airplane, guess what? He and Rhea are assigned to seats right next to each other. (I know you didn't see that coming.) After she demonstrates 1) her anal-retentive nature (I mean, really, who actually cleans off their plasticware before eating those awful airplane meals?) and 2) her complete lack of interest in him, Karan, unable to believe the evidence of his eyes, tries to strike up an acquaintance anyway. They end up spending a few hours together in Amsterdam, which leads to the best song in the film ("Ladki Kyun"), some mutual griping about the stereotypical pitfalls of the respective sexes, and then Karan completely offending Rhea in an almost irredeemable fashion.
They brush up against each other a few months later, with some well-deserved consequences for Karan, and then don't see each other again till years later, back in India. Rhea is getting married, and Karan's mother is her wedding planner. He doesn't make a very good impression on her, again, and she fails to make a very good impression on me, again. They have a dumb song ("Gore Gore Se") at her mehndi ceremony in which she elaborates on the idiocies of guys who are all looks and no commitment. Rani really overacts during this entire sequence, and I hated the song, so that didn't help my opinion. At this point, I started wondering why so many people love this movie.
But then... the second half happened. And I decided I love this movie. I don't want to give away more of the plot, although it's easily available elsewhere should you care to look it up. Suffice it to say that, as Karan grows up, the movie does too. And it's really worth the wait on both counts.
I think the characterization and the character arcs in Hum Tum are its main strengths. The friendship between Mihir and Karan really rang true to me, for instance--I know lots of people whose best friends are exactly like the romantic object of their affections. The strained marriage between Karan's parents--and how it inevitably affects Karan's relationships--also was very well done, despite a cop-out of a resolution.
Saif's portrayal of Karan is another strong positive. He could've been a total jerk, but Saif is good at making him sympathetic despite his flaws. Karan really is a good guy; he just needs some maturity to bring his strengths to the surface. Despite Rhea's repeated dismissal of him, he's always genuinely glad to see her, and that made me believe in the evolution of their relationship.
Rani really acts quite broadly in the first half, but her restrained, muted performance in the second half as a woman who's experienced the tougher side of life and been crushed by it redeemed my perception of Rhea.
The soundtrack, unfortunately, is weak. Like I said, the first song is the best, and it goes steadily downhill from there, except for "Hum Tum," which has an overly mellow adult-contemporary vibe but is sweet nonetheless.
Overall, though, Hum Tum offers some good messages about treating women with respect and being sure to communicate clearly with the object of your affections. Not to mention some supah-fine Saif moments like the one above. I'd say it's worth every penny, and then some.