It depends on how much you are going to use it. I bought a Vodafone dongle and it has performed well but at the end of the 30 days your unused credit evaporates. I have only used it for odd days away from home. O2 have a dongle which you are only charged £2 for a day £7.50 for a week and £15 for a month. That suits my usage pattern better so when the vodafone credit expires I will replace it with an O2 dongle - £19.50 on their website at the moment.