Overall, I found the series enjoyable, though a bit repetitive and probably could have been summed up in 10 episodes and not twice that amount. The ending was exciting, but as I started on the OAV, it was more like a feeble attempt at a reboot and I have to say I lost interest quickly.
There's no real surprises when it comes to character development, but the battle scenes are creative and always keep you on your toes. Plot-wise, you can guess where certain things will lead up to, which is a bit of a disappointment in that something that's supposed to be an exciting twist falls flat.
I personally am not a huge Mecha fan, but I found myself being pleasantly surprised at how much I enjoyed Code Geass overall. I watched the entire series in one go, and there was a good flow across all of the episodes.
So, if you're in the mood for some light angst, guns blazing, rivalry, school shenanigans, and an easy-to-follow storyline, this would be the one for you.