Aion is/was a great game and I loved it, until the latest patches that ruined it.
I played it for one year and a half and I had lots lots of fun, you must have some kind of group to get through it but it was a fantastic experience for me.
I'll start by saying that I rushed the level cap, it didn't take much for my group to reach it, some days we played something like 15-16 hours straight.
Anyway, the pvp that came out of using rifts to get into Elyos leveling zones was awesome, the fact that people communicated between them to say that there were some Asmodian around their realm made it like an hunt and most of the times we could encounter groups of people so we could fight between little groups.
I loved the dredgion PVP which was a mixture of PVP and PVE, you practically had to fight your way trough with mobs and Elyos to reach the command hall and win by either killing the commander or by score.
What was a bit flawed was the flying system, it gave a good advantage to casters and healers, meleers would go mad by chasing casters flying in the air.
Some air fights were fun just because you had to carefully watch your flight time (cause you couldn't fly all the time on AION) or otherwise the flight gauge would drop and you'd fall and die into the void of the abyss
I think that the game was a bit harsh on the occidental community just because many of my friends and other people couldn't bear through the experience requirement to reach the level cap.
I did not mind and I found it enjoyable but I am a rare case, just to tell you my farming attitude I reached level 75 on Lineage II official when it wasn't free to play.
Instances were well made, with good fights and sometimes very hard (every light armored class would get oneshotted if they pulled aggro), especially the last boss in Draupnir Cave, which was 5 levels above the cap and you could literally take one day off to get through the entire instance.
Another harsh wall was the instance drop rate, it was very very low, which for some people who started with easier games like WoW wasn't acceptable.
TL DR; Very enjoyable end game, very harsh leveling (for non oriental minded gamers), AWESOME ground PvP, OK flight PvP.