Normally I use it as a spam catcher/backup e-mail address.
It did a few things that I didn't like:
I now have an extra toolbar on my IE6 browser.
I have a pop-up blocker that I really didn't want - I use other means.
I have a pop-up blocker that allows pop-ups through. When you delve into the settings you find that there is an exception list. One is Yahoo and all of the other items are MSN based. - SURPRISE !
But, worst of all:
Normally I use Outlook as my e-mail client, directly to my personal private domain.
My incoming mail is fine.
But, all of a sudden my outgoing mail gets stuck in the outbox and can't be delivered. Now, I realize there could be many causes, but since the problem started immediately after the download was installed and I haven't changed any settings, it's pretty easy to come to a conspiracy conclusion.
.... and I'm not one bit happy about it !
Edit: This is the MSN upgrade, not the XP-SP2. That one has it's own set of headaches. If you are considering it, go read their white papers first. The service pack sets up a firewall and blocks all incoming files of any type unless it was specifically expecting them.
Sometimes it plays Simon Says - -
"Yes you may."
But sometimes it doesn't ask and just shuts the door.