I'd say Aion is definitely worth it if you're coming from WoW and wants something similar.
Keep in mind the game is very overpopulated right now, so you are going to sit in a queue to log in , in some form. WoW had the same thing when it launched.
The 500mb is not going to go away and it's most likely only going to get bigger , so if you are remotely considering playing Aion , might as well start updating.
Now, if you were looking for a total fresh MMO, Aion might not entirely be it. It's got all the juicy stuff WoW got, it's well polished, but more emphasis on Open World PvP as far as i can tell. Of course i'd say the SA community seems to be grabbing onto Aion, which on an MMO , is what counts. Alot of people play WoW because of other people they know or the size of the community....i expect this to be the same for Aion.
Also ultimately, Aion is NEW, everyone is starting with a clean slate.* WoW on the other hand, you're always behind and odds are whatever you're doing has been done by 5 million players before you. So from that perspective, also worth it..granted you DID enjoy WoW.
*That should be your main reason for picking Aion over WoW [over community].