I have been using Giffgaff for 6 weeks now and I am quite impressed, I am not interested in talk time just data speed and no download limits. The O2 network seems to be being improved with further investment in 3G infrastructure, I think they are investing in upgrading the 900mhz 2G network to higher bandwidth 3G speeds.
I was in Manchester last weekend around the Media City / Salford Quays development and I was getting 4000kbs+ speeds on my Lg GT540 with Giffgaff sim, but they have spent significiant amounts on the fibre network in that area.
My wife has a 3 dongle that she has used occasionally for the past year, usually she gets decent connection speeds of 1000kbs+.
Might be worth getting a couple of payg sims and testing them out before committing to anything long term.