Best material for GRE? Any debrief from GRE test takers?

I took both and I will write-up some of what I learned at length in a couple weeks. But until then (and until you take the test), the most important thing to do is learn vocabulary. It's something that you can't cram (at least I've never been able to) and it will suck up your time.

At least 25% of your scored exam will test your knowledge of often obscure English vocabulary words and how they relate to each other. If you get an experimental verbal section, it will be at least 33% of the questions you have to answer under test conditions. Many of the sentence structure questions will require your knowledge of obscure English vocabulary words so you can correctly parse the sentence, pushing the numbers as high as 3/8 and 1/2 of your final score and test condition questions, depending on how the sentence completion questions are posed.

If you are studying for the GMAT, you are covered on the quant side as far as material; you'll just have to understand the different question types on the two exams. Same with the reading comprehension questions, although those are presented almost identically on the two tests.

So the biggest thing you'll have to do is study the vocab. Then study the vocab again.

Make it a goal between now and your test date to memorize the meanings of at least 10 new words every day. Grab the Kaplan flash cards for the 500 most-tested vocabulary words. If you don't regularly read a high-level news site/publication (NYT, WSJ, Economist -- preferably the Economist because they easily use more GRE words than the other two combined) you should try to start. And start looking up every word you can't immediately define and for which you can't recall a synonym & antonym.

Best of luck and I'll post more info/tips over the next couple of weeks as stuff winds down at work for me.