I have been using the Verizon iPhone with a hotspot plan since it came out. I find it works for browsing regular webpages with text and pictures, but I avoid any video content. I sometimes do a rare download such as a small crucial update for Scrivener (PC version) or an mp3. I have not gone over my 2 gigabyte cap so far and I have found the service reliable.
99% of the time I use the cable tether instead of the wireless connection, because it drains the battery really fast.
What I would check is postings online about other people using the carrier with your particular plan/phone to get an idea of what you are getting into. Also consider the pros and cons of mobility vs stability.