JJ and Dent aren't all that similar. They're both big, and big servers, but JJ is a whole lot better off the ground, and he comes in when he needs to, not as a matter of policy. Unless my memory is fudged, I don't recall JJ being a serve and volleyer.
I also remember his backhand being "solid," like stated above, but the technique wasn't looking all that smooth. I wonder if Agassi can deconstruct that thang.