Should you have stopped at your Master’s degree? Derek Lowe on his blog discusses whether it's worth getting a PhD.
As he sums it up, it's easier to get hired as a non-PhD, but more difficult to advance very far. While PhDs can advance, they have a tougher time landing that hard-to-come-by job.
What's worst, says Lowe, is for a PhD to leave their degree off their application just to be hired and risk being found out. The comments section also gives insight into why people stop at an MS.