B747ERNG It might be fair to say that only you yourself will know whether you need to do a preparation course. If you think that you're a bit weak on a particular area (comprehension for example) it might be worth paying the exorbitant cost of one of these courses.
There are various books which you can get - some of which are mentioned on these threads - and I think they provide good preparation at a greatly reduced cost. Get your hands on a couple of these and just remember that you're not expected (by far) to ace any of the tests... simply put, it's not possible.
Good luck!