These shuttle vans and minibuses can provide a useful service at a competitive price (much cheaper than ataxi, for example), but you must be careful: levels of service and convenience vary.
Before using an airport shuttle service, you should get information on the people providing the service, and its quality:
1. Ask at your hotel about the shuttle service's quality and passenger satisfaction.
2. Ask other travelers if they have used a shuttle service, which one, and how it was.
3. Reserve your shuttle ride through a reliable travel agency: one you know, one that is recommended to you by someone you trust, or one of TTP's recommended travel agencies.
Here's a true airport shuttle horror story from a TTP traveler:
"Here is how it went: the driver came on time, and had a [foreign] family with two young boys in the van; he then picked up 9 more people in hotels (although one seemed Turkish and known to him). It was hot and there was no air conditioning. He then proceeded to go back near the agency, leave us in the car, and return with 3 men, who traveled standing right in front of my seat. I did tell him 'this is dangerous and illegal, you cannot do that' and he answered the only English words he seemed to know 'I am the driver' (in the tone of 'shut up, crazy woman'). He then took the coastal highway at 120-140 km per hour, never braking, only tooting his horn if someone crossed too close, with 18 people in his car of which 3 standing. Even the young American backpackers were rather scared."
If you plan to travel between Atatürk Airport and Sultanahmet by Metro and tram, as you walk from Customs toward the escalator to the tram station, a guy will come up to you and try to get you to take a shuttle van into the city. (It happens to me every time.)
I tell him İstemez! (eess-teh-MEHZ, "It's not wanted").
I don't know who he represents, or whether they provide good service. Another problem with a shuttle van from the airport is that it circulates through the city going to lots of hotels. If yours is the last hotel, you may ride around for quite awhile just at a time when all you want to do is take a shower and relax.