Extremely harsh. Everything I'd say about them is actually included in the guidelines for their use, so there's no point listing it out. I did try to use them, but found that, while Race/Class/Level encounters are the norm in my campaign, monsters are common enough to make using G&G just plain harsh. However, when not using Huge or larger monsters, they worked fairly well.
I eventually settled on a system about half-way between it and W&V.