According to Politico's interview with Cuellar, the congressman uses his iPad first thing in the morning to check the papers while enjoying his cup of coffee. He uses his tablet again in the evening to check his email and see if there are any important news stories.
In terms of professional apps, Cuellar uses Congress in Your Pocket to keep tabs on his fellow politicians. He has another app that works like a teleprompter (unspecified, but likely to be ProPrompter) allowing his aides to send him important talking points. The app he doesn't have? Angry Birds. Apparently, Cuellar likes to keep his iPad productive and distraction-free.
That's not all. Cuellar confirmed that he's "seen a couple of other members that have their iPads." Maybe he's starting a Beltway trend.