[Sombre]

Rat ogres are pretty good I feel - they totally mince up medium/light troops, they can handle high def troops such as cavalry (not knights) better than most skaven and they are quite unlikely to rout. They are best used as either hold and attack shock troops or as flankers.

If you go blood with skaven you can also summon them in quite some numbers - far more effective than going for the one from a master moulder each turn (those are just for shoring up armies, not really for mass production). Bear in mind that they are stealthy (like the rest of your armies) which is rare in such powerful units and they also have a fun graphic ;]