Works very well for GA comics. There aren't many sales of graded books out there for a majority of them, so many resort to the guide to price and sell so the guide ends up influencing the market which then influences the guide.
Harley Yee, one of the best known vintage sellers in the business priced according to guide on all the books I purchased from him. When I worked at a comic shop, we sold silver age and bronze age on e-bay and to save time, we just sold at guide price. It was by far the quickest way to get things done. And when you are paying employees to list stuff on e-bay, time is money
The guide also helps you spot out really quickly which issues stands out from the rest in a run that you might not be aware of.
The guide is just inaccurate for keys and hot books, which is just 1% of comics out there, at most.