I have a MiFi from 3 (Huawei E585, the one with the monochrome display rather than the coloured LEDs) on PAYG and it works great all over the Tyne and Wear area. You will find that if you are anywhere rural (a village or small town) not near a major transport route, that you'll get no 3G signal, and as it cannot fallback to any 2G network, you're cut off completely (short of hunting out a nearby business with WiFi access for customers, or wandering around a housing estate in the hope of stumbling across an unsecured wifi connection).
For me it is no substitute for a home broadband line as I do tend to eat lots of gigabytes quite frequently and there are no mobile broadband plans which offer unlimited data. Certainly not unlimited data for around £10 per month. Another £10 per month for 1GB on the move is nice to activate occasionally if I'll be away a lot.