I am in the market for an air card. I am going to be away from home and need to connect to the internet. I was hoping I could do this with a smaller cell phone or blackberry, but the cost seems prohibitive.

Does anyone use an air card (PCMIA card that connects to a cell phone tower to provice internet connectivity)? How well does this work?
What is the total cost per month to connect this way? Is there normally a set amount of minutes you are allowed (like with a cell phone) before the price increases?
Any hints appreciated.
TIA!