It depends on what everybody means when they say "difficult."
Becky is quite right in that on the Beat the GMAT forums there are way too many questions that focus on small math tricks and the tiniest points of grammar.
If you are talking about Verbal, the OG gives a good level of difficulty on sentence correction. Critical reasoning is good as well although it will help to do some more difficult questions from another source if more practice is needed. Reading comprehension is a little short of passages that are very dense or very long or both. You might want to get another source for a few more difficult passages. Just make sure that you understand every question in the OG.
As to the quantitative the OG does a better job with one kind of difficulty, sneaky difficulty where you are literally funneled toward the wrong answer by the question - often data sufficiency. The other kind is the upfront difficulty of tough concepts. There are all sorts questions concerning coordinate geometry, pemutations, probabilities, etc. on Beat the GMAT forums, these questions test the "math" skills of test-takers. But don't underestimate the sneaky difficulty of official questions.
Overall I would say that too many test-takers think they can memorize their way to a good score. The GMAT is difficult because it is not something that can be memorized.
By the way, if you do well on the OG questions, you should be in a good position to score over a 600. Try the GMATPrep Test and see where you are at this point...
