I respect them (I've never run an ultra race). It's just a different kind of race. Of course if you're running 100 miles there's going to be a variety of paces and finishers - probably more so than something much shorter like an 800m or 1500m. Some people will walk part of it. Some won't. The guys in the videos linked on this thread are clearly running.
I don't really get the disdain for it. Is it because people are frustrated that it isn't as competitive as shorter distances so they get annoyed that people are picking up medals, positions, money for things they don't see as worthy? People seem so aggravated by it.
I think some of the accomplishments are amazing, for example the performances in Marathon des Sables.