I totally disagree, at least for PR (I have no real experience with Kaplan
and have always preferred PR). I am targeting 700+ and am not at all worried that they can't help me get there. I've used their books before, and the strategies worked well for me -- and I had a 3.83 GPA in college, so I'm no dummy who can only score in the 500-600 range. They have a stat on their site that shows that 13% of students who take their course score 700+ on the GMAT, vs. 7% of all test takers. That's not too shabby. And they do group classes by ability and initial diagnostic score, so if you're capable of hitting 700 (a class can't totally do that for you, you know), you won't be sitting with someone who can only score 600.
What matters most, though, are your own ability and dedication, and how much you practice. I can't imagine that Veritas
' or Manhattan's strategies are so superior to PR that if I bust my butt (which I'd do anyway) I can't do just as well with PR as the others. Any of them will help, but it's up to you to make it happen on test day.
Sorry for being verbose but I haven't had my coffee yet!