I am on 3 and have been for around 2 years, signal is terrible around sackville road but better as get to old shoreham rd. Not the best in worthing but I still use it ok for calls and internet. As you go along the coast towards rottingdean and particularly saltdean it is perfect. Hope this helps
I am not from Brighton but I often visit. I've never had a signal problem anywhere there. Its good in Eastbourne or indeed in any major built up area I've visited. Awful signal at my parents but that's at Ripe out in the sticks.
Signing up with the above link would also get you this:
£150 cashback on ANY handset contract (Text & Talk plans), including the The One Plan (inc the iPhone), with all you can eat data, 2000 minutes and 5000 texts. Hurry, limited time offer! Exp 24 Jul 2011
Not interested in 24 month contracts, I've got an unlocked iPhone 4 so I always go for 12 month SIM only deals through quidco and change providers every year