* Prices may differ from that shown
This isn't just useful for those "on the go", but in my own circumstance it's worked out great. I don't pay the phone bills in our house, the person who does, doesn't want the internet and as I was unsure how long I'd end up staying here anyway, this seemed like a good solution - because I do need some internet access!
You can get 3 gigabyte PAYG sim cards for £9-11 on Amazon, the price varies as the sellers keep trying to outdo each other. This is definitely the cheapest type of card I've seen for this modem, it's even cheaper than the 12 gigabyte yearly one. It's also cheaper than any kind of contract from 3g, too.
I've worked out that the above 3 gigabyte sim card, which lasts for up to 3 months, gives you an allowance of 30mb a day over that time, which is fine for checking email daily and occasional surfing. I normally end up using one of these cards a month, though, which means I'm using about 100 megabytes a day. This gives me a few hours of surfing a day, and occasional YouTube videos.
Be warned, lots of videos, music and BBC iplayer are right out, they take up too much bandwidth, sadly. This is only a short term solution, as I find it a quite restrictive.
I'm taking off a star as I'm a little worried about how long it will last, it heats up a great deal. I've had it nearly a year now with several hours use a day and it's still going, though. The service from 3g is usually good, occasionally it goes down completely, but what internet service doesn't? Although it might depend on where you are and how good your coverage is locally.