For all search engine traffic and most referral sites it is, however traffic stats for Stumbleupon for example is generally quite skewed in Analytics due to the frame they use.
Google analytics like other services is accurate, They are inserting a code in your site that is a counter and which helps them to analyse and give you complete report of your site.
It is best tool for taking traffic of a website and i get a better idea to improve traffic on my site with organic keywords.
While other tools can definitely be of use, Google Analytics is definitely something you can rely on.
You will be able to see your traffic and other stats only after 24 hours of installing the tracking codes.
Google Analytics is the #1 resource or source for tracking traffic. I am sure their traffic accuratecy is 95 to 99 percent correct.
It is the best tracking service out there. And it is 100% accurate according to me. It is more accurate than the traffic report produced by your web host.