Excellent sci-fi series with a lot of great practical effects and attention to detail. The characters were all great (goodies and badies) and the story was captivating and mysterious.

Season 4 ruined it for me. It was the opposite of to the above (the upside down :P). The episodes were far too long leading to boredom. They introduced new characters which didnt advance the plot and left some already established characters with nothing to do. On top of this did deep character analysis of the existing characters.