We used StatCounter to monitor traffic to our site. It gave us detailed information on where our traffic came from, what pages were visited, and what browsers were being used. I liked StatCounter because it gave more specific data by visitor than I could get with Google Analytics. In many ways, it is easier to use than Google Analytics, but I used both hand in hand.
StatCounter is a more basic tool than Google Analytics, but in many ways, it is more robust. It provides more detail by visitor. It also allows me to check Google Analytics stats to see if my data matches.
I like it as a backup and indifferent source to check against Google Analytics. It helps me to check on AdWords click fraud and get a better sense for how my site is performing. It is a good basic tool. Google Analytics is far more robust, but Google does not show all of the detail that StatCounter does.