Yes we insure our property with Garanti Bank not sure of the pros and cons it was just easier as long as you have enough money in your account you dont have to do anything they just take it out every year, never claimed (touch wood) so no help there.
Can't comment on their attitude towards paying a claim, but I know several who have had issues with the in house insurance at Garanti.
Renewals can be a problem - some have asked that the policy be renewed and found later it wasn't - others have asked that it is not renewed but Garanti continued to take money from their account. Getting the money refunded was a long and tiresome process.
In addition, Garanti are generally not that competitive; you'll normally get a better deal and better terms from a local broker.