Istanbullus (the residents) are bike friendly. Traffic and the city itself are absolutely horrific for bikers. There are a ton of massive hills, cobblestones, amped up taxi drivers, and mean dogs.
Still, every year there seems to be a ton of bikers here- mostly folks biking accross Eurasia or on a specific bike pilgrimmage. They report that the Asian side is a LOT less hilly and easier to bike on, but generally use the city as an opportunity to rest their legs.
If you NEED to bike on your vacation, consider checking out the Princes' Islands, where the only traffic is other bikes and horse carts. This is a pretty famous area for bicycle exploration, and they can easily be rented near the ferry ports.