done that, and trust me, the most hideous job you can have. You have to move as fast as possible to finish all the tests and find all the bugs and pass to the next stage. And if you find an almost random bug, you have to try to explain it to the stupid programmers who can't understand you, and most of the time they aren't even close to you.
And boy, you panic when you don't find a bug .... (trust me you want a bug, because if you validate that stage of the product, and at the next part someone find a bug in your part ......)
Plus you have confidentiality contract of..... security etc.