The accessibility is OK, not great, but the amounts are really nice. It will take you a long time to mine out the duranium in Mars.
This is an important point to keep in mind from a cost/benefit point of view. Automated mines cost twice as much as regular mines. An accessiblity of 0.5 means that you'll only get minerals out 1/2 as fast as if the mine were situated on a planet with accessibility of 1.0. So if it's Duranium that you're worried about, your return on investment for mines on Mars is going to be 25% that of putting manned mines on a habitable world with duranium accessibility of 1.0.
That being said, it's awfully hard to find colony-cost 0.0 worlds with lots of 1.0 duranium, plus you have to pay the costs of colonizing the world (transporting workers to man the mines, paying for infrastructure if it's cost 2.0 rather than 0.0, etc).
In my games, Earth usually gets mined out after 10-20 years, by which point I need to find a habitable world with good duranium to which I can move my manned mines. The alternative of converting them to automated mines doesn't work very well, since you don't have any net increase in mining capacity while you're doing the conversion. So all of the new mines I build are usually automated - any manned ones I need I simply transfer off Earth, putting off the day that it's mined out. The location of the automated mines I build is usually controlled by where ever I can find the best accessibility (and at least a 5 year supply) of which ever secondary mineral (e.g. Neutronium or Mercassium) I'm running out of, along with a decent (0.5 or greater) accessibility of Duranium.
Just trying to give you an idea of the calculations that go through people's heads when deciding where to put their mining colonies....
John