The Apple SIM in your cellular-equipped iPad is about to become useful for much more than choosing a carrier when you're getting started. Otono Networks has launched AlwaysOnline Wireless, a service that gives you data (up to LTE) while traveling in 45 countries. What you pay depends on both where you're going and whether you'd rather pay by the day, hour or megabyte. If you only need just enough data to catch up on email, for instance, you can pay as little as 99 cents for an hour that includes 15MB of usage.
You can only sign up for AlwaysOnline in a handful of countries, and you can be sure that it'll get expensive to use your tablet's LTE like you would back home. This also won't work with an Apple SIM that has been permanently locked to a carrier, as AT&T likes to do. However, this could be the easiest way to hop online when you're abroad. You only have to activate service when you genuinely need it, and you can do it from the iPad instead of having to call the carrier or otherwise bend over backwards.