Keeping track of my pipeline is the most important piece that Salesforce.com does.
By allowing me to track all of my correspondence with customers and prospects, it enables me to be much more efficient for follow ups. I don't have to go back to pages of hand-written notes to find info about our last conversation or meeting.
It's the de facto standard for CRM. I can't imagine using another CRM solution out there. Additionally, so much of our ecosystem internally is tied to SFDC (i.e. commissions, sales intelligence, etc.).