When it is released it should be faster (provided it does have the 5.X GHz clock) than the 3900X. Intel will have no answer for the 3950X. AMD knows that and has priced the 3950X as a result.
I have a feeling that $749 price will be another "jebait" situation and land at $599 or $649. I have a feeling AMD is waiting for them to release at $749 and drop the price on the 3950x. Hence the September release date.