I spent some hours and found it very usefull. The first steps are well documented, but after the developer ask you to buy a book from lulu, wat I really find a pitty.
Six usd for a download file is not a lot, but does it make sense? I have no problems to pay for a printed book, I also have no problem if there is some good basic documentation to go to a store and to buy a book but if there is any good documentation at all I feel alone on a new planet. And yes, I stopped walking through the new Gantry world exactly for this reason.
And yes, I know how difficult it is to work on an Open Source project, I manage myself a bigger one. But asking money for the core documentation of an Open Source project is simply stupid.