For me, it's free, performs better than yahoo, hotmail or any other well known free web based service I've encountered, catches SPAM better than those stated above and when it came out, I just started using it and got used to it. I despise the newest layout for Yahoo, which would probably be my second choice and hotmail will ever be an option for me again.
From my experience it's also the most secure of the major web based services (if they can really be "secure"). I like that I can look and see the IPs that have been used to log in.
So, cost, ease of use and layout are the most important factors for me and why I chose it. I couldn't care less about google "storing" content or using content to generate keywords...since if anyone really wants to get into an email account (read: the government), they will anyway.