Not accurate at all.

Downloaded the app based on the good reviews. Ive used it for a few months now with no problems other than the fact that it does not accurately show stats. For instance it showed 2 unique visitors on my site while the actual google Analytics site shows over 40. Ive been tracking this for a few weeks now and it has severely undercut the actual numbers by a HUGE amount. I will be switching to a new app shortly. Great looking UI but thats about it.

Anthony Sestito about Fast Analytics for Google Analytics™, v1.2.1