This bus takes you from the airport in Istanbul (Ataturk) right into the center of the area…read more(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 uber as it's much cheaper than a taxi in Istanbul and there are a lot of uber drivers.