[greenyoda]

I agree that domain knowledge is essential. Without domain knowledge, testers may spend way too much time testing insignificant scenarios which a user would be very unlikely to perform, while missing obvious problems that every user would run into.

Overall, my experiences with testers has been positive. As a developer under constant time pressure, it's very easy for me to make stupid mistakes that end up causing grief for users. Since unit tests (or any tests written by a developer) can only test scenarios that the developer has actually thought of, it's good that someone else is testing the product from a user's perspective that's not biased by preconceived notions of how the code should behave.