Speaking from my experience as a skeptical agnostic whose friends and acquaintances were almost uniformly like-minded, I'd say the criteria most non-believers use to judge the general fitness of a fundamentalist as a member of the human race is Are their stated beliefs in agreement with what is held to be good and true in our present society, as expressed in the public square?. Thus, when encountering a fundamentalist either in person or in the media, their conscious or unconscious checklist might run something like, "Do they subscribe to the theory of evolution? Do they support gay/women's/children's rights? Do they believe that all religions ultimately say the same thing, and are therefore all true in some sense? Do they hold single parents and unmarried couples to be equal in status to the nuclear or extended family as an ideal? Do they believe the things all rational, intelligent people clearly ought to in this day and age?"
A Christian who is forming an opinion of a given fundamentalist must use a different criterion: Is this person serving God faithfully?. Such questions are a dangerous business indeed, because whenever a Christian asks "Is this person serving God faithfully?", they are at some point forced to ask "Am I?". For example, it's easy for me to feel superior to prostitute-frequenting evangelist Jimmy Swaggart. But Brother Swaggart spent a lifetime working hard to spread the Gospel, bringing many people to faith in Christ. Can I say as much for myself? There are times when I can hardly talk about Jesus at all in mixed company without blushing at the thought of how medieval and foolish I must sound to my educated, tolerant, broad-minded friends. And even if I had cause to boast, am I so upright and spotless that I would never succumb to temptation as he did?
Thus I tend to take a dim view of fundy-bashing on the part of moderate or liberal Christians, because whatever faults they may possess, in God's sight we are equally culpable in some area.