Last week, the Huffington Post had this link up on its Science section, nice to see something other than the anti-vax idiocy. I quickly recognized it was only the old ion-engine design. This was used recently with a probe or two but it has never been thought of as practical with passengers. The story does mention the biggest factor holding this back is a power source.
I think propulsion is a lower priority with going to Mars. Keeping the crew alive from all of the Sun’s dangerous byproducts (radioactive flares) our atmosphere normally shields us from is the biggest unsolvable problem, something recently brought up by NASA’s current boss Charles Bolden. Personally, I think the first mission to Mars will be a small town full of people going and the plan will involve them staying there for at least a year.