Each of the factions are unique in their own way. The Warhammer entries capitalize a lot on the variety of the factions, far more than the earlier entries; no two of them are at all alike.
For the Skaven, I haven't played them yet, but on the campaign map their settlements appear as ruins to everyone else and they rely a lot on ambushing. For their units they have a lot of weak and cheap units to throw at the enemy (and their deaths don't give morale penalties either) along with smaller contingents of highly dangerous units like warpfire throwers, rat ogres or doomwheels. They also have abilities that can summon clanrats anywhere on the map to disrupt formations. Never discount their wicked artillery either.
In a pitched battle, on the other hand, the Skaven won't last. I can't give more informed view on how their specifics have been achieved, but I think the general consensus has been extremely positive.