You don't need to be very smart to do well on the GMAT. You need two things: hard-work, and perseverance. You need the former and the later. I bet that most people of average intelligence, like me, give up because they lack perseverance, even though they worked really hard.
Look, over the past 3 days, I started taking the practice tests. I started with the Kaplan tests from the Premier Program. I got a 540 on both of them. I was about ready to go kill myself out of despair and frustration. Today I took one of the GMATPrep exams and got a 630. That's a 90 point difference in one day.
Now, I find it very hard to believe the explanation was that I simply had a good day, or that I had very bad days when I took the Kaplan tests. I concede that the explanation may lie in that in part. But, a 90 point jump cannot be solely attributed to a good day or a bad day. That's BS.
What I think, is that Kaplan's tests were harder. They were for me, I don't know about anyone else, but I really struggled with those tests. That's just me. When I took the more "official" GMATPrep test (which is made by the people who make the real thing), I found it MUCH easier. Especially the verbal section.
So, as far as practice tests go, ESP. the Kaplan ones, you should not put too much stock in them. Just keep studying harder than the next guy. Keep taking the tests, keep doing the books and re-doing them and re-doing them. It's 10:40 pm here, I have work tomorrow, I just finished a test, and I'm going to hit the books when I sign off here. You gotta work harder than anyone else. That's the only way to get what you really want (unless you're one of these A**hole geniuses that "decidses" to take the GMAT on a whim tomorrow and gets a 750...they had it handed to them at birth...the rest of us have to earn it).
Like Randy Pausch said (the Carnegie Mellon Professor who just died from cancer), they put brick walls up to keep the people who don't want it badly enough out. You gotta want it more than the other guy. If you are of at least average or moderate intelligence, you can beat the GMAT with sheer, insane, obsessive, brutal hard-work and perseverance.