Phenomenal. There are probably a few more movies with the same idea—a collection of short stories from one person’s life leading up to the present—and Slumdog Millionaire is up there with Forrest Gump and City of God. If you liked either of those, you’ll like SM. And I don’t know anyone that’s seen those movies and doesn’t like them.
I’m not one to complain about subtitles (sometimes I’ll turn them on for domestic movies), but it’s worth praising SM for how seamless the subtitles are. They’re moved up next to the person speaking instead of all of it being centered at the bottom in white. A decent portion of the movie was in Hindi, but I never felt like I was consciously reading anything. The casting is perfect: good child actors, gorgeous actress, and an Indian Jeff Goldblum (I looked it up and I’m not the first person to make this comparison). Watch it.