Life is pretty good at adapting. If we did a lot of selective breeding and genetic manipulation ahead of time and then made a sustained effort to repeatedly seed the more temperate locations on Mars we might eventually see something survive. I do think it's a bit early for that stage of terraforming. We really should experiment with smashing asteroids into Mars to see if we can kick up enough CO2 to make a difference first. I also like the idea of repeatedly impacting a single crater in order to excavate a very deep and wide hole to trap atmospheric gasses and create enough pressure near the bottom for a more human friendly environment. It'd have the added benefit of depositing a bunch of metals and other elements in a convenient location where they could be mined and smelted.