It depends what you are looking nku for but Istanbul is a fabulous city - especially if you like history and culture. There are so many places of interest to visit and many of them are quite breathtaking.
A visit to the Hagia Sophia church/mosque/museum which was built in the 530s is a must. There is also the Blue Mosque and the Suliemaniye Mosque where you can visit the graves of Sulieman the Magnificent and his fascinating wife Roxelanna.
The Grand Bazaar is a wonderful place to explore. It's a covered shopping centre of 80 streets with 3,500 shops and kiosks. It opened in 1461 and it was the largest undercover shopping centre for 600 years. Many of the shops have been owned and run by the same family for centuries. There's also the Spice Bazaar and the Animal Bazaar.
A visit to the Topkapi Palace is unforgettable. The highlights are the Harem as well as the Treasure Room where you can see the breathtaking Jewels of the Ottoman Empire. One set of rooms has many relics of the Prophet Mohammed on display such as his cloak, staff, bow, whiskers etc. That's a fascinating and moving area to visit and be amongst awe-struck Muslims.
You can also cross the Bosphoros on a ferry to Asia and enjoy lots of walks around the city, along the Golden Horn and along the Bosphorus. The Turkish food is superb too.
Hope that helps but it does depend on what you are looking for. If you want a beach resort where they all speak English then maybe Istanbul isn't for you. But if you like history, culture, exotic sights and smells then you'll love it.