I have used '3' on my iPad since day one. The advantage of '3' is that, as their name suggests, they have only a 3G network - so no EDGE or GPRS. Other legacy operators still have a significant proportion of their network basestations - particularly outside city centres - in 2G, which means GPRS and EDGE and very low data rates. Because '3' were a late entrant to the race, they didn't have any 2G licences, so they were never able to deploy 2G basestations and hence their network is all 3G meaning high-speed downloads everywhere - usually the 3.5G HSDPA service of several megabits per second, compared to GPRS and EDGE on, at most, a hundred or so kilobits per second.
Plus, '3' do an 'all you can eat' tariff, so no need to worry about how many GB you're using...
Tim