Mars is the friendliest body in the Solar System for colonization aside from possibly the Moon, which deserves the title only for its proximity to the Earth and low escape velocity. The five elements absolutely essential in large quantities for life - carbon, nitrogen, hydrogen, oxygen, and calcium, are all present on Mars. The atmosphere is only 3% nitrogen in comparison to our 80%, but this is enough to support a colonization effort and eventually, with terraforming, even a limited biosphere. In contrast, the Moon lacks substantial quantities of carbon, nitrogen, or hydrogen.
Mars has a gravity level of 0.38g relative to Earth's 1g. This is the closest to Earth that can be found on theoretically habitable bodies in the Solar System, but still, it's quite low, and could give rise to numerous health problems associated with lower gravity levels. At the very least, it would make it incredibly difficult — or most likely, impossible — for people who grow up on Mars to take trips on the Earth, unless they had cybernetic skeletons of some kind.
One of the most attractive aspects of Mars is its atmosphere. It has one, even thought it is about a 100 times thinner than the Earth's, which is a plus. An atmosphere helps for landings - aerobraking becomes possible. It also protects from harmful cosmic rays; although humans on Mars would still need to live in shielded domes, these domes needn't be as thick as lunar domes would need to be. Mars's atmosphere mostly consists of carbon dioxide, but if hardy plants were introduced to the Martian surface, they could begin converting more of the atmosphere into oxygen. Artificial photosynthesis is also a long-term possibility. Nitrates and carbonates on Mars' surface could be broken up by heat rays for atmospheric nitrogen and life-giving carbon.
Recent evidence has shown past evidence of water on Mars, and the presence of a Martian ice cap is strikingly obvious. The first Martian colonies might be located there, melting down water for drinking and farming. One of the biggest concerns about Martian colonization is the relatively long trip - six to eight months each way.
Experiences with nuclear submarines show that people can probably make trips of this length without going crazy, but one Russian test experiment which put five individuals in a small capsule for a year ended in fisticuffs, prompting a couple of people to leave the experiment. Perhaps cheaper launch costs and better rockets in the future will make this journey less of a mental and physical obstacle. Until then, we will focus our space development on low Earth orbit and the Moon.