“Don’t use it” might be a little too adamant, but the new built-in Compare to Previous Year date-range selection may not always be your best option for comparing date ranges in two consecutive years.
For the majority of websites whose traffic is dictated by day of the week, you should instead apply a custom comparison to correspond with the weekly cycle.
This is not just for the benefit of the main over-time graph, which is much easier to interpret when days of the week are matching. Consider also that September 2012 included five weekends, while September 2011 included only four. If your traffic tends to vary significantly on weekends, a straight September-to-September selection without any offset will skew the comparison for any metric.
A previous post displays a date-range comparison aligned by day of week. This approach is still the most useful in many or most cases, despite the selectors for data-range comparison recently incorporated into the Google Analytics interface.