The others are worth seeing, but not as good as ItB.
In chronological (not production) order:
The Gathering was the pilot. You may or may not have watched it before season 1.
Thirdspace is like an extended episode in season 4.
River of Souls is like an extended episode after season 5.
Legend of the Ranges has a different aesthetic than the others since it was made by SciFi instead of TNT. The story is a bit weak, but it has some fun characters.
A Call To Arms is a pretty good movie which sets up the spinoff, Crusade. It has weird music which is an acquired taste, but is otherwise a decent film.
Crusade itself is a bit of a mess, continuity-wise, but has some fun elements in its 13 episodes.
The Lost Tales is a set of two mini-episodes 10 years after season 5.