I took it last fall and did reasonably well...
I think preparation for the test greatly depends on what you are trying to get into. For Grad Schools that want a lot of critical reasoning ability (econ, business school, engineering, etc.) a high math score is very important. The math section is very similar to the SAT math section. The percentiles are very highly skewed (it's taken for granted that those going into science/math/engineering grad school should get a perfect score or very close, so being in the 75% or so is really good if your going into more of a liberal arts school)
The english section, in my opinion is a lot harder than the SAT english. The types of problems are the same (analogies, fill in the blank, etc.) But the words they ask you to use are really difficult to deal with. The overall scores on this section are much worse (i.e. a 640 in math will get you around the 60th percentile, while in verbal it gets you into the 91st)
The writing section isn't that bad at all, do a few practice runs where you get there writing prompts and quickly write a paper. Typing speed helps a lot in this section. So if you're a slow, two-finger typist, that might be something to fix, especially if you're going into a writing oriented field.
Overall it's not that bad, i didn't study much at all (I'm a science person, so I didn't need to study much for the math section, and grad schools don't care much about verbal/writing). I think what you should work on definitely depends on what you want to get into. But for english, study vocab, for math, go through geometry and algebra I, for writing, make sure you can type, and respond to a prompt in an appropriate 5 paragraph essay.
Hope this helps,
~Lyuokdea