How good is the stock fan for the i5-6600?

Solution
Only reason to get an aftermarket fan for that CPU is if it is to loud for the owner. Cooling wise it is fine so try it out you can always change cooler later.
if you are getting a z170 board with it then you will at least want a tx3, hyper 212, maybe an h7. if you are on a budget the drop to the 6500 and get the hyper 212 that is $20 right now. unless of course if your not going with a z rated board then the stock intel cooler is more than fine.