Pros
Very supportive management, great benefits, positive work environment, good work-life balance. ETS is a great place to work and they treat employees with respect. The training for test developers relies on on-the-job-training so it can be hit or miss depending upon who you are assigned to work with. I was fortunate to work with great people who helped with my professional development. It can be a little overwhelming in the beginning since you learn as you go, but expectations are reasonable for new employees and responsibility tends to be increased with experience. In my experience colleagues and management are very supportive and create an atmosphere of collegiality. ETS is the kind of company where you can build a long career.
Cons
The company culture is very change averse. There is emphasis on length of service over quality of service. If you hang around long enough, you'll be promoted. ETS is terrified of losing employees with experience even if their job performance is lack-luster. Hard work tends to be rewarded with praise, but not necessarily with promotions or compensation. Merit increases are generally given each year, but the difference between the high and low end is not very significant and tends to be based more on where you are within your "salary band" than on performance.