In Tiger it just beachballs whenever you start it up for about a minute. It freaked me out the first time because I thought it would never respond again or it was a broken installation. The interesting thing is that when I run Leopard on that same iMac it happens in Leopard too.
In Leopard/Windows Vista you get weird pauses in the middle of a song and have to hit play again or it takes 5-10 to load up a new song when you click on it.
i run tiger on my 1.66GHz mini and leopard on my MacBook.
i have not had any problems with any version of iTunes to date. i do admit coverflow is a bit slow sometimes but i never use that any ways....