Does Magic Res even work?

As an ADC should i even bother building Mr? cause i still get 1 shot by AP burst even if i do a crazy anti-mage build, and say go Maw and Banshees...i'm still taking so much dmg from champs like that it feels like i'm better of going straight crit every game? I feel like i never stay alive long enough to make lifesteal work now, and mr doesn't do anything...so straight crit seems to win me games more often.