Because I remembered this little known movie fondly from late-night cable years ago, I recently purchased and re-watched it on DVD. My overall memories were a little blurry, but I still like it a lot. It's a good, suspenseful thriller; well told, well paced and well acted. Particularly good is the acting of Eric Foster, who plays David, the young protagonist, as well as that of the actress who plays his sister, Kim Valentine, I believe. It's a little bit dated but remains a classic in its genera.
Really, it boils down to four reasons to watch or like a movie of this type: A good plot, storyline or concept. Good actors/good acting. Unique special effects. Lots of blood/action.
Grandmother's house has virtually no special effects or traditional slasher-type action, and very little blood, but these things are not needed here. I rate it five stars because it scores 100% in both remaining categories. Yes, there are a few weak moments in the film, as there are in any film; plot and acting, but overall the story is solid and the main actors do a very good job.
To me the only thing which can really make a suspense movie truly good is that it actually be suspenseful. The best feature of this film is that it keeps you guessing and uncertain right until the end, without lulling you to sleep along the way. The good acting is a bonus.
If you like horror/suspense movies, (emphasis on the suspense in this case) chances are good you'll like this one.