As I write this I am on said USB donlge provided by three (3).
As my mobile phone is 3, they did a promotion not so long ago whereby they offered the dongle to existing subscribers at half price line rental - so for my dongle I pay £7.50 a month with a 3gb limit on usage - there were higher packages available up to 20gb per month for £15gb (half price).
It all depends where you are as to what speed you will get, for true broadband speeds you need to be in an area that supports HSDPA (Known as 3.5g) which is most well populated areas.
Whilst it will never match the capabilities of true broadband the 3mbps that I am achieving is way more than good enough for your average usage. However they have quite high latency's so its not brilliant for online gaming if you ever do that.
I personally have this as a backup to an broadband internet connection as my line of work requires me to be connected on the move and at home. I have just moved so are using it till BT get their rear ends into gear and activate broadband on my telephone line.
What I would suggest is looking to replace virgin altogether. My personal experience when I lived in docklands was that to replace the tv service alone with SKY+ HD worked out around £1 less per month for the full package including sport and movies - way more channels of junk for less money.
Okay you will have to pay for a BT line if you dont already have one but you can get the basic broadband for free and upgraded packages quite cheaply.
If however you want to be able to have broadband on the move then the dongle is the way to go - it is also great if your not a high usage internet user. Dont forget if you want to go this route that if you go sign up at the carphone warehouse or curry's for an 18 month contract they will chuck in a basic laptop for free as sweetner.
If you have any specific queries feel free to ask cause as I said I am on one right now.
Cheers