I'm in two minds about the GoM. On the one hand, they felt like a breath of fresh air to me, simplifying older, more bombastic methods, and I still like the commonsensical things Damon Brand had to say about the HGA in "Magickal Destiny". On the other, their main appeal seems to have been that their fans went "Could it be that easy?" and bought even more of their books. Their USP seems to be "Magic that will require no groundwork!", and I've come to think it's a flawed premise. I've worked through "Magickal Protection" and still perform the Sword Banishing Ritual every day; I did the whole year of "Success Magick" but these are areas where it's difficult to establish whether their magic has been effective or not – nothing major has befallen me, life has been comparatively good so far but that's about it. However, Brand's and Blaise's shem angel stuff did nothing for me, and the same goes for GoM copycats like Corwin Hargrove and Ben Woodcroft.
"Could it be that easy?" Apparently it can, for a few fortunate ones who perhaps have a natural aptitude for magic. All others, including me, would be well advised to establish some preparatory exercise routine (for me it's currently empty-mind meditation) like the first few stages of Bardon's IIH or Carroll's Liber MMM. That's me troubleshooting my magic, others may draw different conclusions or choose different approaches. "Could it be that easy?" You won't know if you don't try, that's the problem. I'd rather turn to chaos magic in an emergency, that's for sure.