The Bite-Sized series of articles is meant to take the most powerful cards in Dominion and give a short explanation of what makes them powerful.
Tripling Actions is very good, as you may know if you’ve played with King’s Court. While Mastermind doesn’t offer the same craziness, because it doesn’t like to triple other Masterminds, it’s still very powerful. Costing only $5 allows you to put Masterminds in your deck far earlier, which changes a lot about the way you tend to build. Plus, the art is quite nice.
More discussion of Mastermind can be found on this podcast episode.