> Does anyone involved with enterprise purchasing decisions know how valuable Gartner/Forrester endorsements are? And if being in the unmagical quadrant is terrible?
> I see companies tout them but I can't tell if they're effective.
They are very important; their models shape how enterprise thinks about a domain and their recommendations basically make sales. The contents of their expensive reports goes into countless slide decks within the Global 1K.
So working with them (as with the parallel comment from someone at Gitlab points out): you have to be on these companies' decks, which means you have to do outreach to them and shape the technology landscape for them.
They are cheaper than hiring McKinsey who will write a report for you -- Gartner et al will sell you a report they've already done, and then also come discuss it with you.
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Oh wait, you were asking for the quality of the work? It's better, but not much better, than those clickbait review sites that summarise the vendors' spec sheets.
I had a college friend who worked for one of the "major" enterprise analyst companies. My friend had a degree in French literature and needed a paying job. They could write well, so in exchange for a low salary plus benefits they wrote reports on laptops, CNC tools, servers, you name it. They never actually clapped eyes on an actual CNC mill until visiting me one day at work.