Right from the start this series sets itself into a league of it's own. The intro music is every bit as catchy as the show itself. Excel's uber-cute look in the first few minutes is so dang... well, cute! And the constant switching between normal and super deformed characters is a hilarious staple of insane comedy anime.
The jokes in this show are just plain outrageously funny. You name it, they've done it. The director of the anime and the author of the manga are featured in just about every episode doing the most ridiculous things you can think of. Every episode carries an "experimental" theme they try out, from sports to horror, which greatly helps to keep things fresh.
What really keeps this series going for 26 episodes isn't just the over the top insane humor, it's the wonderful characters and story that keeps you watching and wanting to know what happens next. I particularly love the ending episodes where things get serious and wrap up well. It's one of them series where once you finish it, it will seem like you've known it forever.