Pros
GTA package allows you to study in a PhD program for free. It provides tuition benefits (up to 9 credit-hours per semester), health benefits ($1500 per year towards a one-year health insurance plan), and a salary/stipend of $2300 per month. It gives teaching experience you need if you plan academic career. You have to work 20 hours per week, which usually includes 3 hours of attending lectures by a Professor you work with, 4 hours of teaching 3 discussion sections every week, holding 3-6 office hours per week, with preparation and grading taking the rest of 20 hours.
Cons
You have to play two roles simultaneously: be a student on your own, and have your own students. You take your own midterms and grade exams by your students during the same week. Sometimes it gets more intense. This experience may not be as relevant if you plan a research or a consultant career in a private sector. The salary / fellowship benefits will stay the same for 4-5 years of your study. Also, you are not guaranteed that your GTA package will be extended, you have to re-apply every year, and the final decision depends on your GPA being above 3.5.