Well, by now you've seen my position on this with my numerous posts. There is tremendous groundwork to be done before colonization, and that is why I am so pro robotic probe now. We've got to find the resources and do the experiments, and improve the teleoperation and automation of our machines to the point that we can largely live off the land off world. We do need to find out about gravity effects and how to mitigate them if necessary, before colonization can even be considered. The fact that we are talking about lunar bases and relatively long duration mars missions without having even done mouse experiments at those gravity levels is ridiculous. Even if we didn't have ISS to attach a centrifuge module to, we could send up a satellite to perform such animal experiments

A lunar "sky hook" is feasible for transporting material off the moon. So, theoretically we can get a lot of mass out of the gravity well. So, I do believe we can ultimately build large space habitats or live off world. It is a matter of bootstrapping the capability. It will only work if can be almost effectively self-sustaining. Just tourism and entertainment--e.g selling travelogues, off world entertainment productions, off world reality show, etc--could probably bring in sufficient income to trade for needed hi tech goods. If moon material could be easily translated into solar cells, then an assembly line might be set up to put solar power stations in orbit and that could be sold. Basic commodities would have to be available either from the moon or in situ (mars or asteroid).

I am doubtful we will have colonies within a 25 year time horizon, but I think with the right approach it could be done. I would like to see it before I'm gone. I know NASA won't while on this path of wasting so much money putting on a (inefficient) show for the public. We don't need another apollo. Let's do the real work. Private enterprise has a better chance than NASA right now, because at least their business plans will be about sustainability and ultimately profit. So, when they bootstrap up it will be viable by definition. And they won't hesitate to sell what NASA gives away for free (media experience, travel experience, weightless experience, etc.).