Furmark generates too much heat in my opinion. I'm running OpenCL compute operations on my GPUs and they're locked at 100% 24/7. This load is greater than any gaming load, but the temps stay reasonable. Yet after five minutes Furmarking, my temps exceed what they do in a real-life full-load situation after 24 hours, so in my opinion, their burn in test burns a little too much.
EDIT: I didn't mean to imply that you had done anything wrong, plenty of people use Furmark and like it, I'm just not one of them.