At this time (2015) they are a scam. The development of technology and solutions for such travel is not, but the selling of trips is, because it presupposes the existence of such technology and solutions. And they do not exist.

When would such a thing not be a scam?

When when facilities existed on the Red Planet to house and shelter humans and provide for their basic needs. When rockets existed in the size and style required for transport.

Therefore, the business opportunity in 2015 would seem to be in those infrastructure developments.   Those would be nanotech fabricators, various forms of 3d printers and replicators, and robots.  If these are not to the required technological level to accomplish the jobs as defined on Mars, then the useful work would be in bringing them to that stage.

This is all particularly true given that Mars has the full complement of elements to produce plastics, composites, and ceramics, as well as metals and metal alloys.