I've been using Yahoo Mail every day for the past week without any slowdowns, both from home and from work. But my bookmark is directly to their https server (the link to "Secure" mode under the sign in button).
You might know that Yahoo was affected by the Akamai downtime on Tuesday, 6/15/04. Akamai's outage affected a lot of companies, including Yahoo, but for some reason, I was able to login to Yahoo just fine.
And me, I'm sticking with Yahoo Mail, even though I also already have a GMail account. I subscribe to Yahoo Mail Plus for $20/year (current price), and even though I'll never use all of the 2 GB storage, it's the built-in POP3 (no client-side proxies needed), no ads, aggressive anti-spam, zipped eml downloading, and multiple disposable addresses (that can be filtered individually from one account interface) that seal the deal. But for free accounts, who knows, maybe GMail is better, if you plan on going over 100 MB of mail that you really want to keep.
There are also a few things that Google is not addressing to my satisfaction. They've stated that they have no comment when asked about tying GMail accounts to Google searches, and they've also stated that deleted emails will remain stored on their servers. If true, I'd rather stick with Yahoo Mail and use Google for searches, so that one activity won't be tied to the other.