Are we talking about dueling where a warlock can change specs & talents? Then sure, maybe we are good. But no way will you 1v1 anything unless you know what class you are fighting before hand.
As far as dueling goes, the only thing that even has a chance against a good demo lock is a mage. I'd say mage/lock are pretty even.
Since the patch, even full malevolent mages can't beat me. So in my opinion, warlocks are the best 1v1 class, especially demo.