You'll find a lot of folks on the Internet despise Hennessey. He has done some unethical and illegal things (i.e. theft from customers and suppliers), and was previously sued out of business for it (as Hennessey Motorsports, not Hennessey Performance Engineering). He additionally had a habit of personally attacking people on the Internet who were either legitimately upset at being wronged by him, or who ran Internet forums and refused to censor negative comments from those who were wronged by him. (ALL of this information is based on things I've read in car mags or on the Internet, NOT personal or even second-hand experience.)
That said, I can't imagine the truck you picked up being anything less than awesome. Given how much fun this thing is with 400hp, 600 should be absolutely ridiculous. And from the Hennessey spec sheet, it looks like quite a nice build. Can't wait to see it with all the added ADD bits on it too, their stuff looks top-notch.