The converted mana cost of the spell is determined by only its mana cost, no matter what the total cost to cast that spell was
Secret Rendezvous For instance, Secret Rendezvous draws you three cards for three mana
Consequently, it should come as no surprise that the quick-to-kill Gruul Prowess deck has clawed its way to the top of the metagame
Its better than Sleep, helps get things killed, life totals dropped, and tapping all of the stuff without vigilance (or that couldnt attack for some reason like Dueling Grounds or having defender.) This is a very strong card with a very strong effect, given that all of the creatures out there will swing, leaving their players open for counterswings
Getting both in one card is even better value, even if it is lower tempo
It is excellent in decks that naturally mill, discard, surveil, sacrifice, or trade resources early