Must admit that I have no interest in Aion at all, seems far too "easternised" for my liking. I know its toned down a little to be more appealing to the western market, but its still got far too much of the eastern thing going on about it for me.
I very much doubt it "WILL" be "THE" alternative to WoW whatsoever. To be honest, if I had a quid for every time I have seen someone spout that line prior (thats the important word) to an MMO release, I would probably have enough money to make my own MMO by now
I think it will do ok, will do infinitely better in the eastern market than the western market, but I can see it doing around as well as LOTRO or Guild Wars in the western market. In summary, I think it WILL be AN alternative to WoW (much like any other mmo), but not THE.
Try the game for youself, i was wrong about thinking war/darkfall would be it but this REALLY is it, its immense.