I would agree, that the first two were really good, and from I can remember of the third, it was also a good little film...you are right, they are not exactly remakes of each other, but rather, are variations on a theme...and a really spooky theme, because the person is replaced with an exact physical duplicate, but whats inside, is so very different, and eerie and spooky...I love those types of films...
I wasn't a fan of The Thing remake and felt that was one area it screwed up. It touched on the idea of your friend being someone else under the skin but was too into disgusting effects to really work that element, which was the best part of the original story.