This is one of those scary flicks I saw in the early 70's when I was very young (six years old, probably), and probably it was the scariest thing I saw at that time. Certainly, there was no other film like it. I really wonder what sort of attitude the filmmakers had when this was being made. Were they giggling fiendishly, thinking of all the people they would scare with these images? (Scaring people was obviously their plan.) Did they think the movie they were making was "cool"?
The action-packed climax is pretty much the last fifteen minutes of the film. The brain creatures attack the people in the house, and pretty soon, bullets are flying, axes are being swung, and brains are being sucked. In my mind, this completely, mind-blowingly over-the-top ending scene seemed to go on forever, like a seriously bad nightmare. It was so repellently real. The creatures have no eyes, and they sort of "sense" your presence electrically to zero in on you, before flying up at you and clamping themselves onto the back of your neck. The depiction of this was pretty effective, and it still surprises me how well thought-out the creature imagery was here. Surprisingly realistic.
It still works, quite well! Go see it.