Not in my experience. There was an issue with my usage (I think a safari bug, my usage went through the roof for awhile for some reason, where one time my 5GB lasted 9 days).
When you actually run out, even when you bought another 5GB, I generally had to do some troubleshooting like turning off the iPad to get it to use the new plan.
The plan is supposedly only for a single time, so there is no guarantee that it will continue to be available. But it was for the 6 months I used it (went unlimited on ATT as a $20/month add-on to my unlimited iPhone plan).