I will give a +1 for Habitat being a great charity. I did my college internship in the marketing dept of their corporate offices.
They are a very well run charity. (at a corporate level - I have never done a build site) They have a great deal of brain power working for them, and it really showed during my time there. They are very efficient. The full time people they employ are totally committed to their cause, and it was obvious that many of them gave up much more lucrative careers for the sake of the cause. They are also 100% legit - the money you donate goes 100% to where you designate it to be spent. Another thing I have always liked about Habitat, is that they dont just give away houses. Those people that receive homes are very well screened and are in real need. Then they are expected to work on the build of their house for at least x-amount of hours.... and they are expected to take on a mortgage (one tailored to their situation) and actually pay for their house - They dont just give away houses, but rather help people get themselves a house.
As far as Carter is concerned, he was really never anything more then publicity for them. It is urban legend that he started Habitat and made it into what it is, but he did not (it was founded by Millard and Linda Fuller). Carter gave them a national spotlight and allowed them to spread their message, but he didnt really do a damn thing as far as actually contributing to the running of the charity. He was basically just a mascot for them.