I'm really surprised to see so many people say Nasus is underpowered!
I've been playing him as my main since he came out, and feel just as powerful as anyone else. You are able to farm and to gank very well, and when he gets enough damage on his items it's scary.
Yesterday I popped his ultimate in a teamfight and got a quadruple kill in ~5-10 seconds. It was the sickest thing I've done with ANY hero.
You move very fast, hit very hard, and can not only farm but can chase people down.
I see too many people maxing his fire spell -- this is pretty bad, as the skill is pretty ineffective against heroes. I have had a lot of success with 1/1/1 then maxing Q, then fire then wither later on. Oh, and Nasus is NOT a tank! Don't play him like one!