Throughout the past decade we've only been blessed with a few great animes (i.e. Ghost in the Shell: SAC, Full Metal Alchemist, etc.), and most of them have only been released within the past five years. Code Geass easily ranks among them for many reasons.
Following the high school student turned warmonger named Lelouch Lamperouge, a.k.a. Zero, Code Geass presents an impressive story of Lelouch's pursuit to topple his father's empire for the sake of his sister. Leading the forces of the oppressed Japanese against the Empire of Britannia as the terrorist Zero, Lelouch is willing to go to ANY lengths to accomplish his goals, and I mean ANY lengths.
Lelouch is given a unique power early on, however, called Geass, which allows him to control the minds of others and force them to do his bidding, up to and including committing suicide. His use of it is very unique, but even with it he has to rely on his wits to win his battles and keep others from discovering his terrorist activities. The way this is done is extremely clever and will keep you on your toes at all times while watching.
Numerous characters make the story incredibly deep, including terrorist Kallen Stadtfeld who is a devout servant of Lelouch's persona Zero, the seemingly immortal C.C. who granted Lelouch his Geass powers, and the Britannian loyalist Suzaku Kururugi who is one of Lelouch's closest friends (yet also his worst enemy). Each personality is deep and thought out, making each character and his/her struggle truly enjoyable.
Aside from story, Code Geass also features great mecha-based combat and incredibly gory violence. It's definitely not for kids, as a lot of people die horribly. But for those of us who are used to such violence, it's a very nice touch that only improves the depth of the anime. The animations are very smooth and each expression is well done.
To put it simply, Code Geass is a definite smash hit among recent adult anime titles, and one I'll be watching for a long while. It's a show that reminds me a great deal of the anime classic Vision of Escaflowne, which is also one of my favorites. I can only see Code Geass getting better and better.