I did enjoy AO very much. In my opinion it is a good sequel. I must recommend to watch the Final Episode right after. it neatly thighs up the loose ends and really gives you the feeling of an ending.
AO has received a lot of negative from long time fans, and I think this is al due to the long wait. the longer the wait the more the old gets romanticized and the expectations for the new grow. watching all in one stretch (during the course of a month) I must say i enjoyed the whole Eureka Seven series and feel like I gained something new for life.
Just be sure to watch Eureka Seven first.