I use it only when I know I'm coming up to a long light.
It does save fuel. This is a fact. I can tell the difference in my trip computer when at idle with and without auto/start stop.
The only down side is that if you are at stop for too long, the ac starts blowing warm air.
2014 991 C4 GTS
Sold - 2013 E92 M3 SS w/ ZCP l ZPP l 2MK l 6NR l 752
Sold - 2009 E92 335i LMB M-Sport
Sold - 2003 E46 M3 LSB SMG