Rule Zero: Underlings is an amazing product that is well worth your time.
The three best things that Rule Zero: Underlings adds to my games are:
1) The ease with which you track great numbers of minions greatly improves the flow of the game whenever large combats occur.
2) The speed with which I can throw together any random encounters the party may happen across is significantly increased due to the flexibility of underlings.
3) The dramatic tension and excitement that my payers experience when I nearly fill the map with underlings between them and their favorite (most hated) BBEG makes Rule Zero: Underlings worth more than the $2.99 that Paizo.com sells it for.