It is more interesting than some people think at first because you can use your phone to go online with your computer. This is very handy if you are out and about with your laptop or you move house and don't have a phone line.
I just have a cheapo vodafone in the UK and I got just over the speed of a dial-up modem - round about 56kbps. But it did the job until I got broadband installed. It was pay as you go though and they charge you based on how much data you download so it's expensive.
If you had a contract phone with unlimited data access and you only used the internet for average use, you could use your mobile phone for all your mobile broadband needs.
Just think, you have your 3G mobile in your pocket and you are sitting in a lecture hall or on a cheap flight with no wifi, you can connect to your phone via bluetooth and use the connection to get online. Note, mobile phones sometimes have restrictions on what sites you can access though until you use a credit card to verify your age.