I thought the prep course was really useful. For us it was three days, and they were exhausting and emotional. They basically take you through a whole lot of issues you need to think about- why children enter the care system, impact of abuse and neglect on development, attachment disorders, how children may act out loss and trauma, post-adoption support, contact with birth families...
The session I remember best, which was incredibly valuable, was one in which we explored the different ways you may need to parent birth children and adopted children - for example, how 'time out' might work well for a birth child but be really damaging for a child who has been kept locked in a room.
Lots of people also find prep course a great source of networking and mutual support - like the adopters' NCT. This didn't happen for me - the others were perfectly nice, but we had nothing in common (not least because they were all in burkhas and I am a Jewish lesbian ) but it was still interesting to meet them all and hear their stories.
One word of warning: you are told to be open and honest on the course, and of course you should be, but it DOES form part of the assessment process. One of our group took issue with the ethnic matching policy, and I do wonder if that might have blotted her copybook.
On the other hand, it may surprise you to hear (it surprised me) that by the time you get on to prep course they are assuming that you will probably get approved (no guarantee, obviously). In fact, they told us that they already had children in mind for some of us (not for me, sadly!).
So enjoy it, learn from it, be prepared to feel exhausted, emotionally wrung out, and very challenged. But I think you'll find it very worthwhile.