If you are going to Benicassim from Madrid, Barcelona or Valencia consider catching the cheap buses provided by Heineken.
I have tried to translate the details from their website http://www.heineken.es/fibheineken08/busverde/default.aspx
They are running buses from Madrid on the 17th July at Santiago Bernabeu at 9:30 Arrival: Benicàssim / Festival at 15:30 approx the fare is 25 euro
From Barcelona they are running buses on the 17th July at 12:30 from Barcelona Plaza in front of Spain Montjuic Arrival at Benicássim Festival at 16:30 approx. Fare 15 Euro
From Valencia at the Valencia Heineken greenspaces Daily from 15:00 pm 16:00 pm
17:00 pm 18:00 pm
You seem to also get some Heineken Pack which includes The FIB Heineken pack consists of: bag, shirt, cap, key chain, mobile carrier tape, magnet Think green Green Bus ticket.
More details here and Book Benicassim Coach Online
Anyone who speaks spanish could you add more details as I tried to convert their pages from google translate.