I have a TMobile pay as you go. It's very useful when we go on holiday. You can top up however much you like which goes to an account that you get redirected to when you start your web browser. Say, I might top up £15 which will go to the account, you can then select how long you want to use the internet for. I might want 1 day so it will take £2 off of the 15 so the next time I want to use it, i will already have money on it.
£15 is for a month which is basically a monthly contract but we wouldnt use it every month, that's why we got pay as you go.
The only downside to dongles are that if you dont get a powerful enough one then picture could be blurry and video streaming is limited.
Just a bit of extra info ;)