If you find yourself near a Borders Books, Starbucks, etc. it works very well and I find worth the money even though I have wifi at home. I pay the 29.99 month to month when I need it. It's a bit less if you sign a 1 year contract. You can also pay $6 per hour (or portion of), but that will add up fast if you use it more than a couple times a month.

As for your mobile phone- there's good advice above- also check with your provider if they have "tethering" charges. Basically whether they charge extra if you use the phone as a BT modem. It's different than if you send and receive data (web, text, photo's, etc) on the actual phone.