As the most of the commentators argued here, this film has some serious flaws which make it very hard to comprehend. The story line is inconsistent, the acting disjointed and inadequate, not to mention that it represents a new conceptual whole, essentially detached from its predecessor. Still, it has certain qualities - some original visual effects, photography, great atmosphere (especially the scenes in Africa).
Also, put in a broader context, this film manages to capture a part of intellectual ambiance of the late seventies, introducing ideas and concepts that were then considered pseudo-scientific and fanciful, only to become legitimate subject matter of serious research, two decades later. The collapse of humanity into one group mind (obvious references to de Chardin's notion of noosphere, drawn by father Lamont), the parallels between insect (locust) and human society regarding the spread of destructive/violent behavior (check mass psychology, research on swarm intelligence, the popularity of Steven Johnson's 'Emergence' etc.)
So, for those of you expecting horror movie chills and thrills - you should skip this one. But for those of you interested in how quirky scientific ideas inspire pop-culture pieces like 'The Heretic' - it is highly recommended.