Breathtaking.
B-K was one of my first Nintendo 64 games. I was much younger the first time I played it, but it was a magical experience. I came back to the game about a week ago and played through it, 100%. At my current age I am able to truly realize what a marvelous game it is.
Think Super Mario 64, but with more humor and a generally better flow. By flow, I mean the feel of the game's "weaviness". You can really weave through the game without a hitch, unlike SM64. In SM, you get a star and then wind up back in the castle. With Banjo-Kazooie, you get a Jiggy and remain right where you are. That helps you get into the game, and want to play more and more.
That's just one of many of B-K's charms. Another kickass feature was the ability to learn new moves. Unlike SM64, you gained a badass arsenal of showstopping moves in Banjo-Kazooie.
I compare the game to Super Mario 64 because it feels a lot like it, but does a better job. If you play Donkey Kong 64 (another amazing Rare game), it feels VERY similar to Banjo-Kazooie. Maybe that's why I like both a lot.
I'm not sure what it is about Banjo-Kazooie that possesses me every time I play. It could be the huge, original levels. It could be the tacky dialogue or the beautiful, GODLIKE music. It could be the fluid controls and fun gameplay. I dunno what it is, but I love the game, guh-haw!