Transmetropolis

Pros: prompt, fluent English, internet & iPads in vehicles, helpful, professional, luxurious, new local business
Cons: needs to be arranged in advance
Good for: business travel, tourist travel, groups, hotel-​airport shuttle, local knowledge of Istanbul.
I just got back from my friend’s wedding in Turkey.
The bride arranged airport pickup because the airport is far outside the city, I’d never been to Istanbul before, and I wanted to make sure I could land and start doing things right away instead of struggling to figure out the local transit.
Aytac (pronounced «Eye-​Touch»), the owner of Transmetropolis was waiting right outside of customs with my name on an iPad, and called my name as soon as I walked out the door. (He looked up an online photo!)
Aytac helped me exchange money and get me a Turkish number for my cellphone before we left the terminal. His driver, Bahadir («Ba ha») was waiting for us outside in their luxurious custom black Mercedes van. After 16 hours of travel it was so nice to relax in the cool air conditioning, and email my family from their iPads to let them know I was safe and my new Turkish cellphone number.
On the drive, Aytac and Bahadir were very candid about where to visit, and what is going on in Turkey when I asked.
Two days later, Aytac & Bahadir picked me up on their way to the airport to meet my girlfriend. On the way to the airport I saw Aytac using his phone to track where her flight was and the estimated arrival. While we waited for her to get through customs, Aytac told me how long people usually take to come out, and which flights have been arriving early recently. (I think my girlfriend’s flight landed about 20 minutes early)
I really hate taxis or shuttles that are late or unreliable, so it was a big comfort to see how important it was to them that we were early. It took me several days in Istanbul before I figured out the public transit system, and some of the transfers were still a little tricky, so using a shuttle was the way to go.
My friend, the bride, used them to arrange other pickups and drop-​offs as well, and the rest of the group also commented on how nice the pickup was (whether they just slept or wanted to learn about local culture during the ride).
Overall, our experience with Transmetropolis was smooth, easy, and very comfortable. Relaxing with them helped me hit the ground running the afternoon after long international flights, instead of struggling to find a taxi or use public transportation. Their English was superb. Born in Turkey, Aytac went to school in the United States. The driver, Bahadir, also spoke German. I was impressed with their prompt service, English, the internet and iPads in the vehicle, and knowledge of Istanbul.
Edit: I should mention that their van is bigger than a Taxi. I think maybe up to 8 passengers could fit, so this would be a good option for a small group dealing with the «larger than one taxi» problem.