In his talk today, Musk presented a number of interesting and very useful ideas. I don't think they are practical in the form he presented them, but with a little modification, they could be made practical and very powerful. He's right on the mark about using methane/oxygen propellant, which can be made on Mars; about making the spacecraft reusable and refillable on orbit.
The key thing I would change is his plan to send the whole trans Mars propulsion system all the way to Mars and back. Doing that means it can only be used once every four years. Instead he should stage off of it just short of Earth escape. Then it would loop around back to aerobrake into Earth orbit in a week, while the payload habitat craft with just a very small propulsion system for landing would fly on to Mars.
Used this way, the big Earth escape propulsion system could be used 5 times every launch window, instead of once every other launch window, effectively increasing its delivery capacity by a factor of 10. Alternatively, it could deliver the same payload with a system one tenth the size, which is what I would do.
So instead of needing a 500 ton launch capability, he could send the same number of people to Mars every opportunity with a 50 ton launcher, which is what Falcon heavy will be able to do.
The small landing propulsion unit could either be refilled and flown back to LEO, used on Mars for long distance travel, or scrapped and turned into useful parts on Mars using a 3D printer.
Done in this manner, such a transportation system could be implemented much sooner, possibly before the next decade is out, making settlement of Mars a real possibility for our time.
Dr. Robert Zubrin
President & Founder, The Mars Society
President, Pioneer Astronautics