This finale was important viewing to finish the series. It wasn't exactly a 'special' in the sense that the Christmas Specials were discreet episodes whereas this was the last chapter of the whole saga. It tied up the ends quite well, if rather predictably.
I do think it should have been included as part of the sixth and last series rather than an expensive money-spinner in effect for the series makers. Much as I enjoyed seeing the end of the story, I'm cynical about this technique of adding on bits and pieces to make more money. Still, it's what I've come to expect of the BBC.