Transforming versions of villains was a great option for rares and mythic rares, but it was important to have something to feel villainous at lower rarities
Then its worth playing
Prosper, Tome-Bound requires considerably more setup to use optimally, but if your deck is already playing a lot of cards that let you play cards from exile, the value it generates from its Treasure tokens can be absolutely silly
Pile on your cheapest and favorite equipment and aura spells, support them with Impact Tremors effects, and profit
Punch-Clock Villain: Gafgarion - he expresses distaste for what his employers are doing, but he's loyal only to his contract
The fact that many of the cards within, such as Summons, are so well designed only adds to the sets appeal and overall success