We started using google analytics back when it was under its original name of Urchin. A lot has changed since then, not least with this software.
Its free! Not many places you can get access to essentially enterprise level software for a zero cost.
Infinitely customizable. There are numerous websites and manuals available which can teach you ways of turning GA into an almost bespoke solution for your business.
Google continues to innovate this product on what seems like a weekly basis. In recent months the push has been heavily towards social media analytics.
Steep learning curve for the uninitiated.
Bias towards google adwords when it regards advertising tracking.
Currently offers relatieve weak mobile analytics.