Sitting here not knowing if I should update.
I haven't had any problems with iTunes 4, fortunately. I am kinda bummed that if I update to 4.0.1, I can't tell a friend to listen to my playlist over the internet. As it goes, the more people that update, the less people I can share with. Either way, I haven't really shared much over the internet, just seemed like a great feature to have around. I do share within my LAN.
For now, I won't update.