If I had to name some Must-Watch Anime, Code Geass would definitely be one of them.
STORY: The story is amazing, I love how the characters develop and also the main background in Code Geass. Although there is much fighting I really liked the Code Geass world and even until the last few episodes you won't know what the ending will be like.
ART: If the story of Code Geass wouldn't be this great from the beginning until the end I probably would have stopped watching after seeing the first episode. I didn't like the art in the beginning, it was different than what I normally watch BUT luckily the story is amazing and I watched until the end. After watching the first few episodes you get used to the art and I actually loved it by the middle of the anime.
SOUND: If I had to be honest, I don't remember much of the soundtrack except for the openings and endings. But it can't have been to bad or I would have noticed it more. So I give 7 points for this.
CHARACTERS: They are amazing. Code Geass made it happen that everyone of the important characters could tell his/her own story while the main plot was happening. Every character was complicated but still you understand what they feel.
ENJOYMENT: I absolutely enjoyed Code Geass! There is a good mix of action and conversation. So everytime you nearly started to get bored with all the talk they switched it without problems to an action scene and back.
OVERALL: For me, Code Geass is a must watch. Not only for the mecha genre but generally.