It's that good.
And as if this movie wasn't great enough already:
"Gene Kelly had a 103-degree fever when he danced to the title song. The director wanted to send him home, and film the scene another day, but Kelly wouldn't hear of it. He eventually convinced the director that the massive preparations would be wasted, and to allow at least one take. That's all it took - one take - and Kelly went home to recuperate. Most of Kelly's performance is ad libbed - they only had time to rough out his motion up and down the street to establish camera shot opportunities."