When Google launched Gmail on April Fools' Day in 2004, everyone thought it was a joke. What kind of company would be crazy enough to offer their users 1 GB of free email? You have to remember, back in those days you were lucky if you got 100 MB. More storage wasn't the only reason people jumped to Google's email service though, the user interface was absolutely incredible. Threaded conversations, meaning the ability to see multiple replies in one long list, made email usable again. Labels, instead of folders, made compartmentalizing messages a walk in the park.
According to the researchers at comScore, October was the first month in recorded history where more people visited Gmail than Microsoft's competing email service, Hotmail. They say that 287.9 million unique visitors went to Gmail.com in October, versus 286.2 million for Hotmail in the same period. Yahoo! Mail is in third place with 281.7 million views, but it's in first place if you look at just the United States.
If you're not yet using Gmail … what's wrong with you? Google lets you use your own domain, so if you want to use that URL you bought a few years ago, you're more than welcome to do so. And there's also Google Apps for small businesses. Why go with Exchange when Google's service is cheaper and easier to use?