Anyone done a stopover in Istanbul? I am looking at a possible 12 hour stopover in early April.

Some things I am interested in figuring out:
1) Is there anywhere to lock and leave luggage in the Istabul airport?
2) Is it safe a for a woman to wander around town on her own (daylight and evening hours- I would get in at 10am and leave at 10pm)?
3) Is getting through customs and immigration at Istanbul an easy chore and is it easy to get back in?

Thanks!