Yes AoC has been a dissapointment from most accounts. Lots of folks signing up but with the bugs, a lot of folks left.
Most of the AoC complaints I read blow things way out of proportion. I've levelled two different characters to the mid-40s and in that time I ran into only two bugs. It was a really exceptional game, imo. I appreciate that they intentionally made a game that wasn't WoW, that was violent and mature-themed and had exceptional high-end graphics.
As to how AoC is performing in sales, I've seen Blizzard state that, while a good number of people did leave to play Conan, about 40% of people who left have come back. Since I have no context to put that in, I'm not sure if it's a good or a bad thing. Just how many people do stick with an MMO? Has even Blizzard, as the most successful MMO in history, retained as a subscriber every person that's purchased the retail box? Right now it's looking like AoC has around a 500k subscriber base. Far behind WoW, but ahead of every other subscription-based MMO.
Anyhow, back on topic. I am looking forward to WAR, even though I'm not really interested in all the RvR stuff. There are some very good ideas there, like the Tome of Knowledge or Public Quests, and I certainly would love to play a goblin or orc in the Warhammer lore. My only concern is that the PvE side might not be very robust, with so much focus on PvP and RvR.
The game has the potential to sell extremely well with such a long-established fan base, and the modest system requirements will be a real boon. Ironically the thing that will hurt it most, imo, is how similar it looks to WoW. Most people won't know that Warhammer developed that art style to begin with. They'll see a game that came 4 years after WoW, that looks incredibly similar, and consider it the knock-off.