Being a teaching assistant is a job that most graduate students are asked—pressured even—to do. During my time at McGill, I was a T.A. three times: twice for the introduction to compiler design class (taught by my adviser), and once for a functional programming class. It was difficult, time-consuming, and not well paid, yet being a T.A. was the best job I’ve ever had.
I’ve had many bad T.A.’s when I was an undergrad; absent from the message boards, with a poor understanding of the subject, and confusing in their explanations, they make hard classes more difficult and bad classes unbearable.