I'm running the S3 now. I went ahead and bit the bullet, took the KitKat update and do not regret it one bit.
Will I buy another Samsung device now that Knox is here to stay????? I doubt it. I've got at least a year if not longer before I even consider upgrading though. With the way technology is progressing, there's no telling what will be available when the time comes for me to buy another phone. It gives me chills just thinking about what will be available in a year or 2.
I'm very happy with my S3. The phone has some great Devs working on it and I have a feeling development will continue for some time to come.