Obviously not everything the NSA does is a problem from a Constitutional standpoint, but still... the odds of being ordered to do something I would find grossly unethical are too high for me to want to work there.
Worse still, I could see myself modifying my own ethics over time for the sake of staying in my employer's good graces. How many people can honestly say they would never fall prey to that temptation?
A shame, too, because I'll bet they still have some of the coolest toys around, even in the present age of commoditized supercomputing.