Go to the Carphone Warehouse and ask them to check which one has the best coverage in your area. I had one from 3 and I had no problems with the connection at all. Fast and reliable.
I have virgin broadband, which is quite temperamental, and I get many outages (I work from home, so quite important). I back it up with a 3 dongle, which seems to work really well for me. But I use it in the attic of a house, where the reception seems great. I have tried it at ground level, even in Trafalgar Square, and had terrible results. But the combination of the two works really well for me (although, i should really give up on Virgin because they are so bad and never apologize)..
I had a vodafone dongle which hardly had any signal (ground floor flat), before i moved to virgin there were times where i was offline for weeks because of some minor problem with something on the network. o2 broadband can be a good idea if you have a phone with them as they give discounts.
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