Watched a few days ago the recent Blu Ray release of this obscurity and it was a solid effort from visual effects artist John Coates.
The story line is pretty basic and presented in a calm and slow-burn manner by the director. The whole thing has a good pace and excellent special effects for its minimal budget, but for me the best thing is its music, a swirling bass-heavy experimental/ dark ambient electronic music that sets the tone and gives the movie more atmosphere and tension.
Credibility also comes from the well known first-billed duo (MacDonald Carey and Jerry Hardin, here playing sort of a predecessor of the Deep Throat character he played in The X Files. However, their contribution is trivial at best and the Director's Cut that is included in the Blu Ray omits their scenes altogether giving the movie a more fast pace and less talk.
The rest bunch of relative unknowns (one of them is the director) are the real protagonists that do a generally good acting job throughout.
Overall, a worthwhile rarity that will appeal equally to The X-Files fans and also fans of obscure 70s low budget scifi