And now back to our regularly scheduled topic...
Gmail rocks! In addition to all of the features mentioned by others, there are additional features that are valuable.
For instance, if you want to search on your email, you have the power of Google behind you. I use Outlook at work, and it is connected to whatever MS type server runs it. If I want to find something in my work email, it's a chore with maybe a probablility of .75 success. That's versus Gmail with a prob of .95 of success. Ymmv [your mileage may vary], but I usually dread searching work mail whereas I almost never dread searching my Gmail. It isn't just the ability to search and filter; presentation is as important. Again, I usually find what I want through gmail; I do measureably worse with Outlook.
Also, gmail has "labels", something sorely lacking in Outlook. You can have gmail automagically filter email and put multiple labels on messages if you want. You can also manually add labels or remove them. So what? With labels, you can look at one category of email if you wish. Maybe you get a bunch of mail from a group of friends. Maybe too there are a few messages from those same friends that you want to keep in a smaller list. You can do that too.
Fwiw, I have a .mac account, complete with email service. I use my .mac email account in a very limited manner, and don't see it being all that important to me in the future. I like .mac, and the mail service is probably fine, but gmail beats it.
I use gmail strictly online, and have been using it for a few years. For all practical purposes it's the only personal email service I use.