When travelling around Istanbul, you should of course consider using taxis. Taxis are without question the most efficient form of transport through the city. You will not have to share space with other people, and you will be heading directly to your destination. It’s a great way of getting to places quickly, but you must consider that travelling long distance can often be expensive. This is particularly true if you are not travelling with anybody else, as you will not be able to split the fare.
So if you’re travelling long distance across the city, it probably makes more sense to use the trains or other forms of public transport. Not only will this be cheaper for you, but you won’t get stuck in traffic either. Weigh up your finances and time you have to get to your destination, and then make your decision. Taxis are not just the only option.