I have tried two services. I have a laptop with an aircard in it. First try was with T-Mobile. Their service costs $30/month. Figured I would give them a try because they were the cheapest.
They are the cheapest for a reason. During the 14 day trial period, the service was down for more time than it was up. At one point, they told me they had to do something with my account and turned it off completely for 24 hours. Towards the end of my trial period, I called and asked that they extend my trial period because of the problems I had had and still continued to have and they reluctantly agreed to extend my trial period by one day!
After that I switched to Verizon. Their service is not cheap ($80) but it works reasonably well. Also speed is usually about double of what I was getting with T-Mobile and sometimes even triple.
The other things I found that cost less were billed by the megabyte and if you are going to use the connection at all, that will soon become very expensive.
If T-Mobile is in your area, give it a try first. You have 14 days and if it doesn't work, get rid of it!