The title really says it all. But the long of it is...
I liked Yahoo! Mail. It was fairly fast and straightforward and suited my lite email needs perfectly. Plus I started out with a Y! Mail account, so there was a lot of history and inertia.
When the new beta launched recently I tried it out and immediately disliked it. Not because of any UI issues - Outlook interface is perfectly fine. But I nixed it due to performance. It was sloooow. Whatever happened to Yahoo!'s committment to performance?
Which brings me to Gmail. I'm not in love with the way it does threading/conversations, it's no big whoop either way to me, but it is fast. Really fast.
Over the past few months I've slowly started to use my Gmail account more (firstname.lastname@example.org) but lately I've been having issues logging into my Y! Mail account, so I've made the complete switch this week.
So long Y! Mail. (But My Yahoo! is still my homepage.)