Make sure your deck has multiple ways to achieve victorygive it a better than 1 in 14,000,605 chance to reach the End Game
Compare this to commonly played three mana removal spells like Generous Gift - that leaves behind much bigger 3/3 but can smoke lands too (Ditto Beast Within)
If you are the one attacking, then keep track of how much HP your opponent still has
That's a nice, cheap cost for doubling a spell, and I can see this ability working quite well in any predominantly black sacrifice version of a Grixis spells deck
Zen Takahashi Archmages Charm is my absolute favourite counterspell in Modern
At four mana, the best use for Razor Pendulum is discarding to Masticore (a fellow four-drop artifact) or sacrificing to Krark-Clan Ironworks (hey look, more four-drops!)