Yes, it's about mechs. Yes, it's about high-schoolers duking it out in those mechs. But it's good. While the premise sounds incredibly clichéd, Code Geass has enough twists to make it interesting, even if you think you've had your fill of Macross, Robotech and Neon Genesis.
The series is set sometime in the future, after the Britannian Empire, has annexed Japan and renamed it Zone 11. The Japanese, now known as 11's, are clearly second-class citizens compared to Britannian nobility.
The protagonist, Lelouche Lamperouge (horrible name btw, translates as "dodgy red light" in French), is a high-school student with a dark past. One of the many children of the Britannian emperor, he renounced his claims to the throne as a child, after his father refused to avenge or even investigate the murder of his mother. He now lives under a false name in Japan, with his sister Nunnaly, under the protection of the fallen-from-grace noble house of Ashcroft.
He is bitter and disillusioned when he accidentally (?) comes across the mysterious C.C.who offers him a trade: If he's willing to fight the Britannian empire, she will grant him the power of Geass, the ability to force anybody to obey his orders. It only works once per person, but Lelouche, being very intelligent and a masterful tactician, soon employs his power to become the masked leader of the Japanese rebellion against Britannia, in a bid to avenge his mothers death. At the same time, he also has to keep up the masquerade at his school, where, inevitably, it turns out that his best friend has been selected to be an ace-pilot in the Britannian army.
Now, this may still not seem very original or even interesting, but where Code Geass departs from the norm is with Lelouche's personality. He's not the well-meaning but clumsy hero seen in many similar high-school anime. No, Lelouche is very smart, very competent and very ruthless. And the audience gets to experience his slow decline from righteous fighter for freedom to scheming megalomaniac. You want to like him, but that's the trap. As the series advances, you experience the disconnect happening, but it's done so well that you can't really pinpoint where it happened. On the other side, the despotic Britannians slowly reveal some redeeming qualities, though very few characters are clearly good or clearly evil.
In a sense, it's the way that Anakin Skywalker's crossing to the dark side should have been done: subtle, every step seeming justified, every death a necessary sacrifice, every abuse of his power a means to an end, all the more vile when touched against the lodestone that is his blind, disabled sister, who admires him as a sweet, loving brother.
Add to that some nice action scenes, the mystery of who C.C. is and what her motives are and the usual quid-pro-quos inherent in the high-school masquerade, and you've got a very entertaining and refreshing series that I can wholly recommend to all anime fans.
