Taxi seems to be the easiest and cheapest. The cost was like 40 or 50 TRY, if I remember correctly (to Taksim area). The airport is super easy to get around and through. There is a service that my company used once, but I don't know the costs. It included a golf cart pickup at the gate, pre-purchased visa stickers, exit through the CIP passport control section and then baggage assistance / escort to the vehicle. Not sure of the price.
There is something linked to the airport. TAV Primeclass.
www.primeclass.com.tr.
Again, done both and someone who has been through that terminal alot, it's not worth it to me. I would do it myself and just grab a taxi.