The pyramids were built by professionals, not slaves. Additionally, they are still around 4200 years later. Has any other thing man built lasted that long? The Egyptians evidently got their money's worth from that project -- and then some.
I doubt that the grunt work for building them was done by professionals. But even if it was, it was a colossal waste of resources. After all, I'm sure that the pyramids' constriction cost a staggering amount of human time and energy relative to what we available for more productive use. And I'm certain that they were not paid for with voluntary contributions from the general populace.
The fact that they are still around 4200 years later is certainly amazing. It is certainly a tribute to the skill of the designers and planners, and a cultural benefit to those of us alive today -- who of course did not have to bear any of the burdens of paying for the pyramids' construction. But they represent an awful lot of capital being tied up virtually forever for no real productive benefit to those who were forced to pay for them.
Building huge, unimaginably expensive monuments to the egos of leaders is no way for a society to prosper. There is an opportunity cost, which consists of the productive benefit to society of the activity that never happened because so much capital was wasted on a useless public works project.
Just because the structures are still standing is no reason to think the Egyptians got their money's worth out of the effort. If you think they did, perhaps you'd be kind enough to explain to me from a cost-benefit standpoint how the pyramids brought value to the people who paid for them in blood, sweat and treasure.