I have been using GMAIL from mid 2005 and the moment I started using it I just could not go back to Yahoo! which I was using earlier. Hotmail was not even in the league back in 2005.
Although gmail when it was first introduced was quite different in managing email replies and threads and I know a lot of people who could not just get around the management of the replies. I am not sure it was the AJAX experience which caught me or the loading speed (comapred to Yahoo at the time) but I loved the experience and got stuck to it.
GMAIL is continuously evolving and adding features which are continuously enriching the experience for the users. For example colured Labels, the in-line gTalk chat and the recently added themes are some of the few continuous improvements that have been really useful.
If you are a member of a few mailing lists with a lot of traffic, you probably know how annoying it becomes to skim through all the useless junk and focus on the urgent message which you have got from your family. The coloured labels help me do just that, focus on the messages which are important and read the rest whenever you don’t have anything better to do.
A look into my gmail experience.
Aleem's Weblog 
I have use the gmail it is very good for e-mails.i use it in share my picture and messages. it is a very simple . i like it very much.
In internet market yahoo introduce free E-mail system. People likes it so much. but with the passage of time Google Introduce GMAIL to get market share. And gmail proof very successfully . And GMAIL got 75% of market share for free mail system. I like also Gmail. Because gmail is offering so many services in single mail address. Thanks you