I was a teacher's assistant and it was a fine job. I was going to school at the same time, so that added to the stress and will slightly bias my opinion.
The best part of the job is interacting with the students and helping them learn. I thought my boss was a good leader and made sure we were all keeping up with our work. She was fair to the students and to the TA's.
What I didn't like was the lack of training. They dedicated an entire week to theory-based training that was worthless. By the time it's the day to teach, you'll be stuck trying to figure everything out on your own. Poor students! I feel so bad for them. I hardly knew what I was doing that first semester.