Code Geass has it's issues. Some plot points feel overly forced, and other's severely underutilised. There are huge leaps in logic of certain character actions and stances. Several of the character designs do my head in, and the over use of fan service got tiresome fast.
However it's got one of the best main characters in any medium, and the entire show is as a thrill ride from beginning to end. I wouldn't go as far to say it's the best anime I've ever seen, that would be both seasons of Ghost in the Shell Stand Alone Complex which is just amazing. However for sheer entertainment value, Code Geass is nigh on untouched.
I'm not sure how I feel about a 3rd season. The ending to the second was just brilliant, and perfect way to wrap the crazy mess that came before it. I'd love to be wrong, but I can't see a way they can continue the series without pretty much undoing that ending and everything it stood for