I think a positive review can be a way of showing gratitude. In a profession where gifts and cards fall into a weird gray area, a positive online review can be a good alternative. If useful information is included they can be very helpful to people. I have to admit I use these sites all the time and sometimes they are very helpful. I think it's pretty easy to weed out posts that are either spiteful or too adoring and find balanced informative posts. An old pdoc of mine is reviewed on Google and most of the posts are quite negative. Although I liked her at the time, the reviews that were more thoughtful and not just a rant recounted experiences that were very similar to mine. Had I been considering her today, these common experiences would have been a deal breaker for me.