Google's announcement last week that its free, Web-based Gmail service would now in addition to the existing POP3 and Web methods is fantastic news for everyone, but especially for Mac and iPhone users. The new feature makes Gmail a great choice for those who want to access email on the road but have never liked webmail solutions.
iPhone users in particular will benefit from Gmail's top-notch spam filtering. Since the stripped-down Mail program on the iPhone lacks any spam-handling whatsoever, iPhone users are especially reliant on server-side filtering, which not all providers offer. Some Gmail users thus use it only as a middleman between their existing mail server and mail client, taking advantage of Gmail's settings that let the world see whatever email address you specify for outgoing messages.
This change makes Gmail a practical email solution for iPhone users. Previously, the options included using Gmail's Web interface, which works in iPhone Safari but is quite cramped; or using Mail as a POP client, which defeats the purpose of Gmail keeping all your mail in one place. Google even features iPhone setup instructions, along with instructions for Mail, Thunderbird, and other clients.
For Mac users and iPhone users alike, IMAP support also means you can access your "labels" in Gmail as IMAP folders, and take full advantage of the rules-based filtering Gmail offers without losing mobile access to some of your mail.