It is literally covered in the debris from every kind of asteroid there is, so if there is a valuable asteroid type, Ceres has it. If there is a most valuable asteroid, Ceres is it.

This is amazingly fascinating. Just some basic logic really demonstrates that the act of attaching spectral type to asteroids is a imperfect generalization of what nature actually does.

LOTS of asteroids are thought to be rubble piles, and so far they seem to come in a wide variety of combinations. For instance, there seem to be several "kissing" asteroids that gently combined so that its current form is kind of like a barbell shape. Even the recent asteroid that the rosetta probe visited demonstrates some of this. The big sphere-like regions are likely former objects that combined together.

Ceres might have a large center made by one process, and then accumulated various outer layers from subsequent bombardments. It wouldn't be easy, but it would still be practical to mine all the way to the center of it.

Unfortunately it's twice as far as Mars.

This wouldn't actually be a big problem. The transfer orbit burn isn't all that much compared to other components of such trips (like getting out of the gravity well, circularizing in Mars orbit).

What kills us with the asteroids is the inclination, and Ceres is no exception. It's orbit is inclined 10 degrees above the solar system plane. Just consider that Earth is moving at 30 km/s relative to the sun. To ballpark the plane change maneuver, just say 30 km/s times sin of 10 degrees. We're up to 5.2 km/s burn. That's actually a bigger deal than the distance to the sun due the efficiency of Hohmann transfers.

This is important because it's not just Ceres that suffers this inaccessibility due to inclination. Many "near earth" asteroids are only "near" in the location sense, but not in the velocity sense. If something is right next to use in orbital semi-major axis but highly inclined, the burn is like going to Jupiter and gravity assists are difficult to use for these.