I love Gmail. Without it I'd be lost. I use it for personal email; work related stuff at times; and, use it to manage the deluge of stuff I get as Executive Editor for Technorati. I access it via the web as well as my iPhone and iPad via IMAP.
I like it because it helps me filter stuff seamlessly, it's awesome with killing spam and of course, finding stuff that's been buried for months via Google search.
There are a slew of other reasons, but instead of rambling on, check out these four thorough posts from TiBITS, one of the few blogs I read religiously.
After reading, if you still can't understand the value of using Gmail as your e-mail client, you must be a Hotmail or Yahoo mail user. ;p