If we go by the standard of worldly success, the pagan gods are clearly useless. On the one hand, their greatest follower (Regulus) was tortured to death by Rome's hated enemy, while the wicked Marius achieved the heights of worldly power despite his clear disdain for all that is good. But on the other hand, we also have clear examples of the opposite--devout and pious men like Metellus prosper while the wicked Catiline was overthrown and died in poverty and disgrace.
What we really see at work in the world is not the power of pagan gods, but the movement of the hand of Providence, wherein the Lord directs the affairs of the world as He wills. If Marius came to power it was only at the Divine command, and if he (or the demons posing as pagan gods) could not exercise that power to the full extent his wicked heart desired, that was because of Divine restraint on his sinful will.
Lest we think that because he was opposed to Marius Sulla must have been a Godly man, Augustine points out that he too was on the side of demons in rebellion against God.
We really have a remarkable picture of the nature of sin in this chapter. Whether talking about demons or our own sin, the goal of both is that we all might be involved "in one common wickedness and judgment of God." The sole nature of sin is to set ourselves up against the God who created us and who alone has the right to be worshiped.
Augustine is equally clear about the result of sin: "for by it [Sulla's victory] he became so insatiable in his desires, and was rendered so arrogant and reckless by prosperity, that he may be said rather to have inflicted a moral destruction on himself than corporal destruction on his enemies."
What happens when we pursue some kind of worldly good as our chief end? For Sulla it was power and influence in the Republic (both against its enemies and against other Romans), but for us it could be anything--financial success, marriage, family, national glory, etc. The result of this sort of a pursuit is that even if we get the object our desires, our desires themselves only become more corrupted because they were misdirected in the first place.
By contrast, the Christian (imperfectly to be sure) attempts to reflect the Catechism's declaration of man's chief end: "To glorify God and enjoy Him forever." By grace, rather than descending ever more into sin and rebellion we have increasingly refined affections that are ever more in conformity with God's will and display the fruits of the Spirit that signify a life touched by the saving grace of God.