My experience is far from recent, but when I decided to go to graduate school, I was undecided between a Ph.D. program in biology or an MBA at the time. As such, I took both the GRE and GMAT and got very mediocre scores on both. Once I researched the prospects for the Ph.D. more and saw that it was likely to be a longer, unpleasant, and far more expensive road versus what I was going to earning at the end, I decided on the MBA. I signed up for the Princeton Review (circa 1995/1996), and if nothing else, it provided me with the discipline to study more for the test each night after a long day of work every day. Ultimately, my score went up about 100 points. In my case, I did great at the math portion, but sucked at verbal, though oddly enough the essay was my strongest section.
If you have the time and discipline to study with a self-help book, I’m convinced you don’t need any of these services. Like anything else, the more familiar you are with the test questions, their format and layout, the timing, and the type of tricks they use that make you pick wrong answers, the better your performance will be. Perhaps the only thing where self help might be an issue is the essay portion, but I’m not so sure the Princeton Review people were any better at providing advice and scoring that than a friend or family member would be.