The "Tholian Web" spacesuits looked interesting, with the red/blue "circulatory system" running over the surface. But those eraser-head helmets were just too goofy to accept. They block the user's vision on the sides, while leaving the "view" out the back wide open. (Why?)
How about the advantages of such a helmet design? During extra-vehicular activity the TOS suit allows the user to lift his head and see what's above him with ease.
Maybe the open space at the back is rather a "view-in" for fellow astronauts. In case of a communication problem a guy behind could just say "move your head so I can see that everything is okay".
Unlike TMP the general idea seemed to be to have a space suit that looked so futuristic that it could almost pass as believable. Given the time and budget restraints I think the outcome was impressive.