I did the Academy in 2001. As everynoe else has said already, it was great fun and an excellent introduction to motorsport.
I'm not sure I'd recommend it now though as the price seems to have got a bit silly. As a £10.5k car it was good value, as something closer to £15 now there are other options worth considering. People have already mentioned buying secondhand and this is a good option, There's no shortage of people here and on the Graduates web site who would be more than happy to help you through the first few events till you know what's what. In fact, the folk on the Graduates web site probably gives you more of an idea about what you need to be doing than the support from Caterham for the Academy!
Other options (wait for the flames!) are Radicals, Formula Ford, Mini's, etc. All have their pros and cons, though none have a community of enthusiasts like a Caterham! One big consideration is how tightly specified the various classes are specified. You want to make sure that you don't end up in a class where you can spend your way to success...it's nice to know you are winning (o being beaten) because of your skill rather than the size of your wallet!
Simon