If you can maintain living in a spacecraft - an environment much more inhospitable than Mars - for 18 months (there and back), I'm pretty sure a month or so on Mars is relatively easy.
This conversation isn't about spending time I'm Mars; its about living their indefinitely. Living there indefinitely means either shipping everything out (ridiculously expensive and inefficient) or creating our own Earth-esque biosphere over part of the planet, which is an extremely difficult task.
Mars atmosphere is not that problematic. Melt the poles with nuclear reactors. This creates a CO2 atmosphere. Then use plants to convert CO2 to O2. Now you have a breathable atmosphere, and more heat trapped and thus a more temperate climate. Mars is particularly cool because you can actually terraform it without having to resort to unobtanium. Other planetary bodies and moons locally, not so much.
Without a magnetic field wouldn't this new atmosphere be blown away by the solar wind? If so, it seems a little irresponsible to propose melting the poles. It seems like a better idea to dig deep and build cities under the surface.
I am also no expert, but from what I've read, probably not in the long term. It's commonly believed that Mars used to be more Earth-like, but the deterioration of its magnetic field caused its atmosphere to seep away into space. (Mars' gravity is also just generally weaker than any planet except Mercury.)
