Well it arrived earlier this week (or last, depends upon when you read this). I upgraded too and all I can tell immediately is the change of the interface. There are other features explained officially at www.apple.com/itunes such as preventing objectionable content from playing for children and organizational stuff. I’ll have to play with it more to see if I notice any difference.
Should you upgrade? Most likely because Apple is unlikely to resolve anything ugly you encounter in version 4 point anything.
I’ve used it and haven’t noticed a single difference. Maybe I don’t get into it enough.