I ran a WFRP campaign using D20 rules for a while.
The party got to about 5th level and I had to force them all to change to the WFRP rules.
The AD&D system doesn't support the 'Grim World of Perilous Adventure' - the power level is far too high.
And beyond 5th level, there's no possibility of 'Lucky Goblin Syndrome' creeping in. In WFRP it's perfectly possible for a crummy goblin to critical and kill a powerful knight on horseback.
In AD&D, that same goblin would have to hit and critical about 40 times to have the same effect.
LGS is what makes the Warhammer world 'perilous' - no character is safe from it.
When talking about the resource books (and I think I've got 'em all now), then try to avoid the battle game books. Especially the latest release. It's too colourful and cartoon-y. Warhammer Battle has turned away from the gothic roots it had in the 80s, and is now all about selly garish coloured paints to little kids. As a result, even the skeletons have to be attractive. The world as sold by Games Workshop ain't as grim as it used to be.
The good stuff's in the Roleplay rulebook. But without really re-hashing the HitPoint rules and downgrading spells, the D20 system is too over the top to function properly.
I agree about most of the races, mind you. Skaven are all about the attitude. If you're just interested in Skaven I believe that Hogshead are due to do a Skaven sourcebook at some stage.
Anyway. I've gone on for long enough. Shutting up now...