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.
JF