Havatas

This bus takes you from the airport in Istanbul (Ataturk) right into the center of the area (Taksim) and it stops regularly. It looks like it does have other buses for other stops, but Taksim was most common. It can be found as soon as you leave the airport as there are always several lined up by the taxis and many say Havatas and Taksim on the outside.
It’s approximately 30 minutes from the airport to the final destination in Taksim and about 22 Turkish Lira (from other reviewers it looks like it can vary depending on day) but it ends up being around 7 dollars.
Driving in Istanbul looks to me like an insurance claim waiting to happen. The only thing that worried me about taking this bus, was wondering where I would end up (how close to hotel), so attaching map pic I used to help me decide where to book.
If you don’t take the Havatas, taking the metro would mean changing several trains from the airport and would also take more than an hour. The only other good option (in my experience) is to über as it’s much cheaper than a taxi in Istanbul and there are a lot of über drivers.

Service from Sabiha Gokcen airport to Taksim was 15TL per person and well worth it. A taxi would have cost us at least 100TL.
The Havatas shuttle has a couple of different routes. Make sure to double check your destination with the driver.
Tickets are purchased on the shuttle bus. The shuttle leaves every 30 minutes from 4am to 1am.
I would definitely use this service again.

Easy, leaves on time, friendly service (some of the personel even speak very good English) and tales delays in to account. We used Havatas from Sabina Gokçen to Taksim and back again. The first trip took 1 h 20 min due to a major trafic jam. The trip back to the airport took 45 min. Both within the 1h 30min that they tell you to expect.

Havatas bietet die günstigste Alternative um von den Flughäfen in die Stadt zukommen. Bezüglich der Abfahrtszeiten kann ich nichts verbindliches Sagen, da ich denke, dass diese immer losfahren sobald sie voll sind.
Bezahlt wird an Bord.