Hi Apples,
Coverage is okay in Brighton, and it switches automatically to GPRS when it can't get a 3G signal. Service quality is pretty good when you've got coverage. This time last year I was working in London (City) and had a Vodafone 3G phone: 3G coverage was very good so should be no problems for you on that front.
The main reason I bought Vodafone prepay is for international coverage: I'm going on holiday at Easter and although I won't be actively trading I want to keep an eye on my position trades - this seemed the best route, not wanting to have to keep finding internet cafes. What I've done is also bought a Vodafone Portugal prepay SIM as well as the UK one that came with the card which means I'll be paying local data access fees rather than roaming.
If I were planning to use it more in the UK I would definitely get a contract: £30/mth for 250MB. If you buy contract then the card is only £60 from Vodafone (or free if you sign up to the higher usage tariffs), so no point buying one on Ebay and taking the risk that it's dodgy in some way.
There's not much else to say: set up very simple (in theory - I had a few problems as it conflicted with a COM port, but sorted quickly enough when searching the error message on the internet), and then you launch your applications as normal - eg I just fired up my CMC Markets Forex platform and it worked fine, as well as internet explorer. There's quite a good background utility from Vodafone which lets you keep track of exactly how much data your sending to and fro. All in all an impressive package - although using it 'in anger' on holiday in a couple of weeks will be the real test for me....