Kaplan's Portable GRE Exam has excellent content. I prefer the paperback Barron's GRE: Graduate Record Examination that I also purchased, but since that generally stays back while I carry my Kindle with me, I have used Kaplan's a great deal more. Both of them are far superior to Peterson's 30 Days to the GRE CAT, though in all fairness that work is quite dated at this point. In particular, I think Kaplan does a fine and thorough job of breaking down the verbal section of the test and providing good strategies to attack it. I'm not a big fan of many of their mathematical explanations, but it may just be rubbing me the wrong way since it seems rather dumbed-down.
My main point of contention with this product is the poor job of formatting. Somewhere in the conversion process most of the vocabulary words that Barron's suggests you study became clumped together sotheyformonegiantnoncoherentsentencegoingonforpageafterpage. Not only does this make it difficult simply to read the words in order to see what Kaplan's is suggesting, this also detracts from one of the main advantages of the Kindle version of the study guide - you cannot simply bring the cursor up to look up the odd word whose definition you are unfamiliar with. While you could exit out of Kaplan's to look up the word in the dictionary, doesn't that remove a great deal of the utility of the Kindle itself? One of my original purposes in purchasing this device was to use it for school books and study guides, and now the publisher's poor quality assurance/quality control has made that point moot. So buyer beware until Kaplan's reformats this. Four out of five stars for good content and clear writing, minus one for this major Kindle issue.