One of the best French comedies ever filmed. The basic idea is comedy gold and the execution rises to the challenge : the plot is both witty and funny, the dialogues are inspired and there are puns, jokes and running gags galore, interspersed with gloriously demented scenes. Even the montages meant to show the passage of time are delightful. In short : one recognizes the divine paw of René Goscinny, of "Obelix and Asterix" fame.
And the fun doesn't stop there : the movie has a stellar cast giving stellar performances. About the only thing bothering me is Michel Serrault's make-up. The said make-up, which is supposed to represent the gradual progression of time deep into venerable old age, is both ugly and unconvincing. And this is not an insignificant detail, given the fact that the age and aging of Serrault are major plot elements.
Viewers need to be aware that much of the humor is dark to the point of blackness : the comedy is speaking of the corrosive effects of avarice and envy on the human soul. However, it is not an immoral or nihilistic movie : innocence is rewarded, while the various evildoers all get what's coming to them. In fact, the ending includes a pretty literal case of an "engineer hoist with his own petard"...