I've been twice now - this year and in my freshers year in 2008, and I've found it to be a good evening...for a few hours. The first year I went it rained, and after watching 3/4 hours of the parades and getting soaked, the bonfire sites were washed out and my group wanted to go home, and then we had to wait 2 hours to get back (and this was at 10pm). This year I enjoyed the parades much more, but to be honest, I got a bit bored. The problem is that because you have to get there so early to avoid queues at Brighton station (got the 5.20 train after queuing 30 mins) by the time the procession has gone past twice, it's still only 8pm and the fireworks don't start for another few hours....and then you know you'll have to queue for hours to get back.
Hmmm, I sound like such a moaner!! It's fun, but I don't know, four hours of processions?! Oh and on the way back I experienced the weirdest thing - Brighton on a Friday night....dead silent. It was so strange!