While I'll admit this episode is a bit off for a "first episode", it's definitely not underrated at all. It's quite an awesome episode, it gives you a good feel of the characters, and the actual episode is fairly interesting and fun to watch, but one thing in this episode is that it has some of my favorite songs from the series. I love the song that plays when they're dancing and their version of Bach's piece which happens to be a favorite of his.
Lots of excellent music choices are picked throughout the series and having such good ones in the first episode proves that.
The only thing I really have to say negatively though is if this was ever the actual first episode I saw, I probably wouldn't have liked Kelly as much as I do. She doesn't have as much personality in this, but it really is quite good either way.