As soon as Yahoo shows me my inbox instead of an ad when I sign in, I'd consider going back. Until then, all the wasted time clicking away from the ad cancels any extra uptime. Luckily I've heard Marissa is actually pushing for this change :)
If she wanted it done, it'd already be done.
While you are on that page, enable SSL by default: Turn on SSL: Check "Make your Yahoo! Mail more secure with SSL"
http://fastmail.fm