Bringing this back up for those who may be interested.
We have an old iPad, first gen, with cellular available via AT&T for $15/month (I think 250MB). We used it a couple times on vacation, but not in the last few years.
I ordered the $10 mobile SIM from T-Mobile a week ago and got it today. Plugged it in the SIM slot, called T-Mo to activate (free), and within 10 minutes, it was working. Good thing because our internet went out an hour later, so I could use the free data to "chat" with TWC instead of my cell phone.
Obviously, I don't have a lot of experience with it, but it's free, and has already come in handy. The coverage is limited, as is the data, but for the one time $10 outlay, I'm happy so far.