Energetic, hectic, fun, but be on your toes
One of the most energetic, lively, overwhelming cities I've been. Everyone is in the streets at night, and their conversations join with the shouting of store owners and buskers to make for a festive and hectic atmosphere. The Blue Mosque and Haga Sophia are not to be missed. The museums are decent, but not essential. Get over to the Eastern side too, it's subtly different. The food is amazing. The cafés are excellent. Definitely try Nargile, and the chewy turkish ice cream.
Insider Tip:If you're traveling alone or in a small group, read up on all the scams on obvious outsiders - especially 'free drinks', 'shoe shine', 'carpet seller' and the various techniques of pick pockets. I was approached about a dozen times in the two weeks I was there, and I even barely thwarted a pick pocketing attempt. Also, if you like walking around, consult your host before taking off in random directions. There are some neighborhoods that are just not safe at night.