definately go, had a brilliant time and will be looking to go again next summer.
not much I can add to what's already been covered really. there are three campsites, one is right outside the train station and is the closest to town I think, but I trekked into it on the final day and it felt small and miserable. would definately recommend the campsite we stayed in, the one adjoining the festival site itself. can't remember it's name, someone will know.
also, would echo the 'buy train tickets early'. in advance of the trip if possible, if only to put your mind at ease. we stayed in barcelona for a few nights before the festival, very nearly missed the last train out for days (about 2/3 days solid of barca->beni trains were booked up, according to what we could gather from the station staff). we had cheap return tickets, but didn't really know what train we were getting so packed up early and went to the station. had to sit around for hours, then suffer a horrible train ride back to barca. don't let it put you off, like I say, just prepare in advance to save all the hassle.
as far as I know, it's fairly simple to get to benicassim from barcelona, valencia and madrid, so they're all options if you want a few nights in a city too. personally, I think I'd prefer next time to have the city break after benicassim instead of before. or perhaps both
but yeah, go!