If the signal is good what you need to find out about is the contention, but thats pretty much on a suck it and see basis. I’ve found anywhere close to an office based working population – regardless of how good the signal is for calls – 3G/internet is woeful. The network is clogged with shirking office workers facebooking and tweeting from their phones on the sly. Come 6pm and it can the network functions much better though. Residentially, especially if you are somewhere a bit rent/studenty/night-lifey it can be the other way around and the network is slower in the evenings but better in the day.
You can get a dongle, usually with some PAYG credit on it, for £10-£20 from the supermarket, you just need to try it and see really