i am also preparing for the TEF since it is the cheapest way to get permanent residence since my family is in ontario.(universities are very expensive in ontario)
after studing french for one year and using the ASSIMIL mehods i just started with the TEF preparation guides, it's not that difficult, just like the ielts if you understand english it is only learning the stucture of the test and do some practice. the most important thing is knowing french.
to me the most difficult module is the writing section or expression orale. french can be difficult to write.
however, since the cic stated that new rules for express entry will be announced this fall i decided to wait before i take the test.