I took it a couple of years ago. The general GRE is a breeze - I didn't study and got something like 168 for quant, 164 for verbal, 4.5 for written. You don't need to study at all for it; just read a past exam paper to get an idea what to expect.
For science graduate programs they care a lot more about the subject test. I did pretty poorly on the maths one (770 - 77th percentile) but apparently it was good enough to get into Princeton, so meh. If you can do some previous papers, then by all means that's a good way to prepare. Of course, the GRE is a very small part of your application (I don't think any place is going to care if you got 98th percentile or 88th percentile, for example, though less than 80th percentile is not impressive if you're applying to top tier universities). Most care more about letters of recommendation/GPA/statement of purpose/research experience.
Also, for applying to grad schools, thegradcafe is a pretty good website with an active forum, plus people post their application results so you can get an idea of how good you need to be to get into some programs.