I've been using Gmail since Google employees posted their first invitations to sign up for Gmail on ebay selling those invitations for around $5.00. I've never had a problem with Gmail, ever. It isn't perfect by any means, but an account is yours forever no matter who your ISP is. Here's some reasons why you should use Gmail instead of ISP mail.
ISP mail isn't yours forever. When you switch from AT&T to Comcast or some other ISP, you lose your email address. After a few years, people become dependent on their unique email address. A Gmail address is yours forever no matter what ISP you use.
Seven Gigabytes of storage and counting. Gmail's account size is constantly growing and just recently passed the 7GB storage mark. That's a lot of space for mail.
Gmail supports POP and IMAP, when activated, to support mail delivery to email programs such as Thunderbird, Apple Mail, Outlook and a host of others. Frankly, not having all those emails filling up my hard drive by using web mail gives me a lot of peace of mind. Additionally, I don't have to worry about backing up important emails, Google does it for me.
Gmail works with the iPhone. Gmail is already part of the iPhone, and there are other applications such as Google Talk for the iPhone as well.
Google Calendar is a fantastic and simple calendar application that is part of Gmail. It's easy to configure and use. It can be synced with iCal, Outlook or viewed as a web page. You can even embed a Google calendar into your own website.
Google Notifier displays new emails and Calendar events without having a browser launched. It lives on the menu bar and notifies users of new emails that have arrived and new upcoming calendar events. There is some configuration involved, but it is very easy to use.
That's just a few reasons. Google has many online applications available for both Mac and Windows users. If you're looking for an alternative to your current email service or a little extra, give Google a try.
Erik