I've been using Gmail for a while now. The capacity has grown from 2GB to 5.4GB, and I've only used 9% so far. Google Apps is so appealing to me that I intend to transfer my father-in-law's church to it once I start to administer the network.
The truly amazing thing about Gmail is the free POP3 and SMTP access, and now the inclusion of IMAP. The difference between POP and IMAP is how email is stored; in POP, you take the email away from the server, and in IMAP you duplicate what is on the server.
Some ISP's have offered POP for free, and most charge for SMTP access. None of the other free email hosting services offer IMAP at all. Google is a pioneer in that regard.
With POP email, the mail is moved to your computer; if your computer crashes, the email is gone. With IMAP email, the mail on the server is synchronized with your computer; if your computer crashes, the mail is still on the server and available to you. If you delete a message from the server, it is gone from your computer. If you mark a message as read on the server, it is marked as read on your computer.
If you have used corporate email like Exchange or Lotus then you understand client/server email. IMAP is a great way to pick up email on multiple devices without you having to keep them all in sync.
If you are still using an email like Yahoo! or Hotmail, or YourName@Verizon.net (or sbcglobal.net, etc) then I would encourage you to get a gmail account, or better yet, get your own domain and migrate your mail server to Google Apps. If you need a gmail invite, just drop me a line.