if you're planning on using GS as your only prep material, DON'T. i bought the 10-CBTs (b/c i've already exhausted the kaplan F/Ls the first time i was prepping for the exam), and I swear it would've been more beneficial for me to just redo all old tests I already took... i've had a bad experience with each GS test i took (technical problems, etc) and have been wasting time trying to look up explanations in my other prep material to the explanations they give for test answers. so IMHO, don't waste your time... spend a little more and get yourself some concrete test prep material.
good luck!