I spent about 2 hours running Warhammer last night. This is in Windows XP, now using the latest 180.44 drivers, playing at 1280x800 resolution.
Running around in Altdorf (known to cause problems for some), I had no issues. I was running in Balanced mode, fps was around 25-40, I'd say on average it was fairly stable around 30. Game looked great!
Later I pretty much just played "normally". Ran around, killed mobs, etc. Stayed pretty much on target in the 30fps range. Some times I'd get a little stuttering with special effects or new areas loading in, but nothing abnormal.
Key responsiveness was fine. I have a set sequence when farming groups of 3-4 mobs at a time, and it's critical that the timing is correct or I'll be toast. I had no problems with this at all, so that was definitely good news.
I also q'd up for some scenarios. Here's where I had more stuttering. I was able to play through but it was a struggle/slideshow at some points. Particularly when too many graphics effects on screen at once. I switched to Fastest Framerate mode and had some improvement. I should mention though that even my decked out desktop will have problems on some occasions, so this is not a shock.
All in all - the game is definitely playable. With some tweaks I can probably get better performance. I don't expect to be running this in big open RvR/keep battles, as I suspect that will certainly be a limitation. For soloing, general grouping, and even scenarios, the game is playable for me. YMMV.