This may be old, but it's happened so gradually it passed me by. Gmail, still in "beta", has morphed into a complete replacement for traditional ISP email services.
Sure it's been a great email client for years -- I've used it since about day one. At first, however, it was a standalone webmail service. Then Google introduced POP functionality (alas, not IMAP, which is what geeks really want). Then they combined POP with archiving and forwarding -- so you can pick up email from the inbox and Gmail will automatically archive it. Lately, and I didn't noticWhat I didn't notice, they've added authenticated SMTP (sending services). The latter passed me by, but it's documented in Gmail's email configuration directions. Maybe Google reasoned, correctly, that distinguishing between POP and SMTP would just confuse everyone -- so they only announced the POP portion.
With spam filtering , POP, and authenticated SMTP  Gmail is a full replacement for ISP email services. Users of traditional email clients (OS X Mail.app, Outlook Express, Mozilla, Thunderbird, etc) who start out with Gmail can switch ISPs with impunity, without any loss of email services . Users who need a very simplified email interface (visual issues, elderly, etc) can use special market email software with Gmail.
Email is a big part of the identity management and reputation management functions that will be fundamental in the next 20 years. Google has stealthily split this functionality from connectivity provision. Clever of them. Now, how did I miss this?
 Not on forwarded email - so beware!. It works well on email sent directly to gmail.com
 Some trash-quality ISPs block the ports used for authenticated SMTP.
 Just forward from the ISP provided email. If Gmail managed spam on forwarded email it would be just about perfect.