What I like is that it's 1.0 FL OZ.'s, so these meet the TSA's requirements, and you can bring this on your carry-on in a clear ziploc baggie. They also don't take up a lot of space and there's plenty of product to last awhile, allowing you to travel light without all the bulk of full-size bottles. The only con is that MAC doesn't let you use your pro discount on these items, and frankly $10-$15 is somewhat steep considering they are just travel sizes, and they are pretty small in comparison to the full-sized ones.
Comparison of size in my hand (nailpolish is OPI "Coquette Cutie")