M.L. Martin
Hero
There's a bunch of concrete information out there. Amazon is pretty generous with information, and compares with other companies.
The problem is that Amazon is 'snapshot' data--how things are doing over an uncertain but apparently fairly short and recent period of time--and purely comparative. It makes it hard to tell how things are doing over the long term, or exactly how well a product ranked at X is doing compared to one ranked at Y.
It's still useful for seeing what's hot and what isn't, but it's not going to give us a very good picture of how 4E is doing over a two-year period in comparison to Pathfinder, or to the 3.5E launch.