Quick one - Visits vs. Visitors is a great piece on why the first matters more than you might think and why the second is less accurate than you might presume. Good read:

Here are my top reasons for using visits:
1) Visits are more accurate than Unique Visitors.
2) Every Visit represents an opportunity to persuade or convert a visitor to a customer.
3) Measuring visits is based on fairly established industry standards

Guess I'm on an analytics binge today.