Gartner has been sued about ranking companies that buy hefty consulting services from them higher on its magic quadrant rankings than other companies that don't. The case got dismissed[1] on other grounds, but the general sense I've gotten anecdotally is that it kind of is a payola scheme.
Gartner is one of those companies that executives hire to provide backing for their big decisions, so that if they don't work out there is someone to blame. Gartner makes a lot of money to be the scapegoat-in-waiting.
While I wouldn't ignore the original poster, these research companies do have a good reputation amongst their primary customer bases. Many HN commentators have opinions, occasionally an understanding of a technology/market, and infrequently have any large enterprise experience, so they don't see the need for services like Gartner.
In the enterprise you can never get a decision wrong which is why these services have value. Stuff goes wrong all the time, but the culture is very much to cover your ass, which is why Gartner have such value.
You'll get most value out of Gartner, Forrester, etc. if you are researching technology in an enterprise scenario and have done a lot of the basics yourself. You can use their write-ups as references to support decisions you are making, but if you have the option you are better submitting an analyst request and having them review your findings or add additional suggestions.
On the vendor side it is a mixed bag. You have a IBM's and other entrenched players in the up quadrant for any market. It's hard to make the leap from being seen as a niche player because their analyses take into account all of the supporting services they offer. Unless you going really deep or plan to offer consulting services etc. then there doesn't seem much point to trying to get onto the Gartner Quadrant. You might be better off the radar completely because then a potential customer can't hear a detailed Gartner comparison which may be too generic to hold you in good light.
IT people outside of the decision-making arena don't see the point in Gartner et al because they know enough about the business area to find and implement the solution. The problem is that they are often not trusted within the enterprise to make these decisions because of the culture and structure of the organisation.
1) If you're a business manager who doesn't live and breathe IT, then their opinion is a good sign-post.
2) If you need "proof" to backup your purchase position, then it can serve as ammunition.
3) If you're a marketer, their blessing is priceless.
However, if you're an IT expert, then their reports may be missing subtle nuances.
Note that the public reports are not necessarily their most valuable products.
For example, I saw a presentation by the 451 Group that literally predicted the future of smartphones 2 years in advance. Still weirds me out how accurate they were.
