Famous kernel developer Ingo Molnar said the following:

Atime updates are by far the biggest IO performance deficiency that Linux has today.  Getting rid of atime updates would give us more everyday Linux performance than all the pagecache speedups of the past 10 years, _combined_.

http://blogs.koolwal.net/2009/01/30/installing-linux-on-usb-part-4-noatime-and-relatime-mount-options/

And here's some of what the L man said about it:

http://lkml.org/lkml/2007/8/4/73

http://lkml.org/lkml/2007/8/10/200

Noatime is good, but remember that you will not be able to know when something was last accessed, so consider your needs carefully. Many people can do without atime, though you must be careful, as some things like backups, the popularity contest, and other things rely on it.

Be well informed!