I have used Yahoo! since 1996 (originally a free Geocities POP3 account), and am currently using a paid account.

I find it much superior to Gmail. Their beta AJAX web interface has the look and feel of a full desktop mail client, and is very similar to Thunderbird. Yahoo!'s AddressGuard feature is extremely useful - Google has nothing really comparable. The only drawback is that Yahoo! does not yet offer IMAP.

I use Google for my calendar and contacts (and use the Google exchange server to sync both of these to my phone).