Gmail is spectacular and part of my shortlist of essential pieces of software.
I've been using it since its inception in early Beta, back when it gave 1GB vs the 2mb given by Hotmail.
Gmail is superbly intuitive, easy to use, vastly available on pretty much every software platform and device and compatible with mostly every email client, secure, fast, allows for large filesizes.
Fully integrated with google's software package and devices, allowing you to instantly sync calendar appointments, calendars and the like between all of your google account.
The Spam filter works spectacularly and very rarely filters out good email, or lets spam into your inbox.
Very easy to search for e-mail, advanced and intuitive formatting options, easy to paste or attach files or fomats, nearly instant sync of sent and received e-mails.
Gmail has been a trendsetter having rivals play constant catch up when it comes to storage, speed, security, and UI. Truly essential and the best free email alternative around.
Easily the hardest "Dislike" column I've had to write; I wish both the interface, formatting and editing of contacts within Gmail were handled better and more intuitively, it's much more of a hassle to use than similar approaches like that of iCloud.
There's barely anything wrong with Gmail.