Over the years, I've found the GS guide to be fairly accurate. But, you have to pay attention to the site descriptions in terms of average width and depth. But, you also have to remember that campgrounds don't get reviewed every year, so things can change. As an example, I just looked up a campground for our upcoming trip and it was listed as a GS campground (eligible for a discount). When I went to the campground's website, they have become part of the KOA network.
As to personal reviews....they are really so subjective and all you can do is weigh the amount of positive versus negative reviews and whether it's only a handful of reviews, or lots of them.
It can help to view the campground via Google Earth. By zooming in, depending on what satellite images are loaded, you can get a better idea of the size of the campsites and the layout of the campground.