I have been using FireStats for gathering statistics on WordPress sites for more than a couple of years now. I mainly use FireStats combined with WordPress Stats, and Google Analytics. Each of them give you different views into traffic, but FireStats is by far quick and fast and give you a good overall picture you can dig down using WordPress Stats and Google Analytics. I just installed the new version 1.6.0 on this blog and found a new interesting goodie. Now FireStats also tracks the number of RSS readers coming to your site :D