I've been a Gmail user for many years, and recently test-drove a few other services, and have come back to Gmail. The one I thought I liked was Outlook.com, and I didn't dislike it. But integration with SkyDrive (their cloud service) wasn't as good as it initially looked to be. Additionally, as a user with multiple computer platforms (PC, Mac, Linux), the availability of Chrome on all OSes was a big plus. As you may know, there is a Gmail Offline plug-in for Chrome, and it works well for me. I like it better than the web interface for Gmail. I also really like how integrated Gmail is with its cloud service, Google Drive. If in Gmail, you can attach a document straight off your Google Drive cloud. I remember having problems doing that in SkyDrive/Outlook. I might not have found the right way to do it in Outlook.
Either way, I've stuck with Gmail because of the very good connection across all Google services, from Picasa (photos), to YouTube (videos), Gmail (email), Calendar, and web. Single sign-on is nice and everything just WORKS together well.
I have no illusions: Google will take over the world one day. I still can't get away from using their excellent services, though.
