The study guides provided by ISA are not really educational (IMO). They are more like a list of topics and concepts that ISA expects you to know in order to pass the test. Their literature is devoid of any differing ideas or information about conflicting opinions to the dogma asserted in their booklets.
Overall, it's pretty good information, and a great place to start beefing up your tree knowledge, and the booklets are a quick way to find the weak spots in your knowledge. If you find that some parts are hard to follow/remember, then get a better book on that subject and really learn something.
Passing the test is not really proof that you are an expert, either. There is LOTS of stuff that I don't know, and I breezed through the test.
How tough to pass? It depends on your background. I have a strong background in higher education, so the test for me was little more than preparation for a semester test for a 3 hour course. On the other hand, if you never took any college courses in botany or biology, the plant biology section will probably be pretty tough, and will involve a lot of preparation.