I have the same card. One of the GPUs went out of it in May. I sent it to Diamond in May. I got an email today that I will get the money credited back to my account. That is the longest RMA I have personally every seen.
I think they must just go in spurts, as literally today I got a notice that my RMA was processed and a refund granted through newegg.
+1 on agreement: the longest RMA process ever.
You can't make generic statements like that based on only 2 or 3 data points. In fact neither of us can: I have had the exact opposite experience with Diamond. I have RMA'd cards with pretty much every vendors (Sapphire, Visiontek, Diamond, HIS), and in my experience Diamond was the fastest (8 days between shipping the defective 5970 and receiving a brand new replacement, that was in August 2011).
The truth is that the quality and speed of the RMA process varies across the world and depending on which employees or business partners handles of your specific RMA request. It would take thousands of RMA experiences to accurately tell which vendor does the best job.