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.
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