1) All contacts and email content is immediately searchable.
2) The 'Star' feature gives you a quick and easy to-do list.
3) It's free and ubiquitous.
4) The 'archive' feature lets you maintain a clutter-free inbox.
5) You can send/receive emails from multiple addresses. (i.e. firstname.lastname@example.org or email@example.com)
6) Virus scanning on incoming messages (again, free).
7) Access from just about any cell phone.
8) Gmail can host your corporate email system (again, for free).
9) Perfect functioning spam-filter.
10) Did I mention it was free and ubiquitous.