In recent years, the GMAT has been trying hard to become more “relevant” to business school. So, a decade ago, its arguments were just slightly dumbed down versions of the LSAT arguments. Now, some of their arguments (but by no means all, see the first worked example for a classic one) have a more “business decision” feel to them. These questions fit less cleanly into the basic structures of the question type, though at root, the rules are the same. However, to smash these questions, you need to have a stronger intuition about how these questions work so that you can apply the rules more flexibly.
As a consequence, I actually recommend to GMAT studiers that you go buy some LSAT tests — they sell individual tests, which might be enough, or books of 10 tests, which, if this stuff is hard for you right now, might be a good investment. These questions are good for building your foundation for two reasons: they have very transparent, predictable structures, and the types of logic they use can, on harder questions, be more sophisticated than what you will see on the GMAT. This builds extra capacity and good intuition. Then you can go work on the GMAT questions with a much stronger foundation.
Test Smashers 
Does anyone even take the GMAT now? I was under the impression that most B-schools now accept the GRE, and so I’m curious why someone would choose the GMAT instead. Wonder whether you have thoughts on how someone might make this decision if their schools permit either test.
This is a great question, since the ETS vs GMAC wars are heating up with the new integrated reasoning section on the GMAT. This question is worthy of a post on its own, but for now, I’ll point you to this recent article: http://www.businessweek.com/articles/2012-04-26/study-few-mba-applicants-consider-taking-the-gre.
Definitely people still take the GMAT. Whether you should or not is an open question whose answer is probably changing pretty quickly. If for some reason you score dramatically better on the GRE than on the GMAT (yes, you should be taking practice tests in both to get a sense of what is at stake), then the GRE is the way to go. At least, it is if all the schools you want to apply to accept the GRE (check early!). GMAT is still the more conservative choice: it has long been considered a harder exam, and even if that is changing, there are still a ton of B-School admissions officers who have been on the job long enough to have a GMAT bias.
An 800 on any test will dazzle, for sure. If you are scoring 580s (or their new equivalent in the rescaled GRE), you might make a better case for yourself if you are submitting a score from the perceived “more rigorous” test.
My worry is that we are entering a competitive spiral: the tests are going to start changing rapidly, admissions officers will start chasing fads, and applicants are going to start feeling like they have to take both tests. As a veteran TestSmasher, I say you should tune all this out, pick a test, stick with it, then smash the hell out of it.