ARmadgeddon has some interesting points about the data that analyst firms (Gartner is the main one talked about, but the point is universal) base their decisions on. The bottom line - it's a thin reed. Plenty of good take aways, but here's a really good thing to keep in mind:

Another issue is that data points are limited to clients of the analyst firms. Gartner’s CEO has stated that Gartner has only 15% of the possible end user market at companies. Does Gartner’s or Forrester’s client base represent a statistically valid sample of the overall IT buyer market?

The client base is also self selecting, which creates problems of its own. So - the question you want to ask yourself is this: Are the answers you get back worth what you pay for them?