I live in a remote valley with very poor internet availability and several people I know have internet with a usb mobile ‘dongle’. It seems to work fine for them. The ones I know use vodafone. It obviously depends on the network coverage in your area, and whether there is 3G coverage.
I tried the same system earlier this year when I was in the UK for about 6 weeks. I bought a pay as you go dongle from the 3 network. It worked fine, and was surprisingly quick. the main problem I found was that it gobbled up the bandwidth I had paid for - until I turned off images in firefox - from that moment onwards it was incredibly cheap. some sites don’t work well - I turned the images back on, used that site and then turned them back off. Skype worked a treat - and didn’t use much bandwidth either.
On balance - I would say it depends on the coverage you have, your usage, if the deal is for unlimited bandwidth or not - and whether you can cope turning images off if not.