Played the beta for both and didn't bother to keep playing after they launched. Aion looks pretty but that's the extent, it's very Asian centric, think Lineage II with the Crysis engine. Champions at first looked promising but it grew tiresome by the time you hit level 20, though the nemisis feature was pretty cool and the art style is also pretty neat but there's really nothing more beyond that.
They are definitely not the WoW killers some are claiming they are. The only reason why Aion has the numbers it does is their ROK player base and initial subscriptions in China due to WoW being yanked for a few months over there. Now that WoW is back in business in China, subscriptions have dropped quite a bit for Aion. Aion will initially do well in the West because it's new and looks nice but you'll see the numbers start to dwindle after about 6-9 months, especially after FFXIV launches next year.
ack-ack