Any of you who have had a look at my site will see that I did the CCC think back in 1992. I also know a couple of people doing it now in the Phillipines. For what its worth, here's my two cents (largly american audience or I would have said pence!)...
I joined CCC as a Science Officer (that's for free!!!). I'm glad I didn't pay. I'll list the bad points first:
1/ Its expensive for what you get in terms of accomodation and living.
2/ If you volunteer (pay), you will have to work. Cook, clean, fill tanks, etc.
3/ The science (when I was there) is not taken very seriously.
On the good side:
1/ You will do a lot of diving.
2/ You will have an excellent time.
3/ You will get very drunk.
4/ You will score (if you try!)
5/ You can justify it to yours parents/CV/peers as doing your bit for the environment.
I would say as something to do during or just after university it a blast. I wouldn't use up my annual leave for it though!
Hope this helps.