Just back from first time in IST. It is no Paris (at least from the non-Turkish, tourist POV), but it is certainly worth 3/4 days. I especially like the Haga Sophia which may well be the oldest (and one of the biggest) buildings in the world. IST is a lot more interesting than Athens.

Keep in mind that Turkey, long famous for being inexpensive, is that no longer. It is now, more or less, part of the Eurozone, at least in the tourist part of IST, and prices are on a par with, say, Spain. Double, at least, any prices you see quoted in any guide book more than a couple of months old.

IMHO, a biz ticket to IST from the US is very good value for miles. It is about as far as you can go for 90K or so. Iberia (AA partner) has good service to IST.