Vodafone Sure Signal works for us.
When we first moved to the barn even GPRS was pants. Now GPRS is better, but in the house we have 3G connection via our our BB line. We dont use a landline to having a consistent mobile reception is very important (we work from home). Only real issue is in peak evening browsing time, if you try and stream a dense video say, while on the phone the call quality can suffer, but its not often.
Sure Signal had a bad reputation initially as the servers could be very flaky. Ours has been fine for at least 12 months now with no glitches so I think they've ironed out any problem.s