I have been using gmail since it was first released via invitation only back in 2004. I still keep my Hotmail account just for junk mailings (if I need to sign up for something). However, I am extremely pleased with gmail and its philosphy that you do not have to throw away anything ever nor do you have to file things away- just search for it. This is a great tool. If you need to review an email with a key code in it or download link, just type something in the search box that was in the email and there it is. It also narrows down email addresses as you type them in (or the name associated with them) in the address field. Once you see a list of names appear, just choose the one you want- no more address book or memorizing email addresses. On top of that you have storage galore and google talk built right in. Did I mention that their spam filter is phenomenal? I don't remember the last time I had spam slip through into my inbox.
In addition to email, your google account also opens up to you the ability to utilize google calendar, google docs, and a plethora of other services.
There are also "add ons" that can turn your gmail account into a storage space for files and such. There is even one that utilizes it to run a web based operating system.
So, I say all that to ask this: who is your email client and why?