I know the default attitude is cynicism when it comes to anything the large FTO's do, however I have to take exception to some of 'the beak's' post! The assessments are not meaningless, pointless, nor are they a complete waste of time. There is a strong correlation between success at the assessments and doing well on the course. If this wasn't the case, they wouldn't offer the 'skills guarantee' (where they will refund all your training costs if you fail), (or they would be making a huge loss from it)!
The assessments are difficult, intensive, and demanding - just like the course. In a lot of ways it is a filter - if someone can be bothered to do their research and revise well for the assessment then it shows commitment. Over the 2 days the FTO will learn a lot about a candidate's suitability - aptitude, trainability, personality, commitment, dedication, inter personal skills etc.
However, there is no such thing as a perfect system and there will always be good people who are turned down because they had a bad day, or people who slip through the net. But do you think the airline selection procedure is any different? It's not. (And is often broadly similar to the FTO's assessments).
Personally, I failed the assessments several times on my road to becoming an airline pilot, but I persevered, worked hard, went back and passed them. I think that showed how determined I was to succeed, and if someone demonstrates that, then that is one of the most important qualities that will see you through your training and your career.
I agree that knowledge is a very important part of training, but you have to be determined, dedicated, and hard working to acquire that knowledge (one of the things being assessed). The assessment also tests some of that knowledge (maths/physics) that you will need.
Adimondo - I would go back and resit the OAA tests (if you do it says a lot about how much you really want this), but work hard before you go to improve on the areas you fell down on. There are books out there to help with spacial awareness, and contrary to popular belief, you CAN (will) improve your scores through practice. I agree with others - go and sit the GAPAN tests, and why not CABAIR, FTE and CTC? Even if you don't want to go there it will be very good practice, and the expense is small compared to the overall cost of training. Good luck!