What do we mean when we say it today?
For someone who hates deception, Hamlet sure finds himself coming up with a lot of lies. Hamlet depicts a seamy political world, where deception is a necessary part of life and political "spin" rules the day. In other words, it's a lot like our world—and like pretty much every political court or congress in history.
When Hamlet says "piece of work," he tells us a couple of different things. First, man is better than any of God's creatures, he thinks. And second, man is far from a masterpiece. He's still missing a few things that would make him perfect.
We take it one step further today. We aren't just subtly saying men need improvement when we use this phrase. We're saying someone is lacking morals or compassion. You definitely do not want to be called "a piece of work." That work is never a masterpiece.