Netvibes introduced a whole new set of customization features. I have been using iGoogle for the past few months because I am in love with Google. I have used MyYahoo, MyMSN in the past too, not lately though. Now with Netvibes giving you the option of adding new themes and wallpapers, I like it a lot. I know MyYahoo lets you add content and has multiple pages, but I like those little gadgets that you can add to iGoogle.
see netvibes theme choices 
I think Netvibes is taking over all of them in looks as well as functionality and ease of use. It lets you cram a lot of the universe on just one simple page, all at your finger tips, hundreds of feeds, emails from loads of different clients, to-do lists, and even chat-clients, all can be reached via your home page. Guess I am going to be using netvibes more than iGoogle these days.
See more at netvibes blog
What do you use and why?
Sephyroth
I’ve been testing out iGoogle, but for the most part, I don’t use a portal page.
I kinda think that My Yahoo! is a bit busy for my tastes, and I’ve never actually visited Netvibes, so I’m not going to pass judgment on that site for now ;)