Another thing to consider is if you need to change terminals. I have found it can take a long time to go from one terminal to another. That underground "train" can be painfully slow when you're in a hurry. The last time I went through Houston (almost 2 years ago), there was alot of construction and the only way to get to the other terminals was by that train. Plus, you probably will have to go through a security checkpoint.

When you return, remember you also have to clear customs and claim your luggage.