Mars' atmosphere isn't awsome. It is nearly non-existent. At sea level, on Earth, atmospheric pressure is about 14.7 pounds per square inch. At surface level on Mars, it's a whopping 0.087 psi. As far as your body is concerned, that's like a vacuum. As far as most plants are concerned, it's like a freeze-drier.
If you raised the temperature on Mars a few degrees, and thus brought *all* the frozen carbon dioxide to gaseous state, you'd get a mean surface pressure about half of what you'd feel on the highest mountain on Earth. The greenhouse effect then takes over, melting water ice and generally bringing the climate to above water's freezing point for maybe half the surface. While you couldn't breathe unaided, you'd not need a pressure suit to go outside - just a breathing mask. And, then you can start bringing in plants - plankton being the biggest winner for O2 production.
Unfortunately, lack of a magnetic field causes some issues - making it more difficult to hold on to an atmosphere, especially one with water in it.
