If Call of Cthulhu had really been made in 1925, it would be widely hailed as one of the landmarks of horror cinema and its makers accepted as true cinema geniuses. The viewer is likely to get more pleasure, the better able they are to put themselves in that imaginary place.
Sure, it'll accidentally jar you out occasionally. A couple of actors don't quite look period enough and a couple, but only a couple, of music cues sound over-modern.
Overall, however, it's like a great silent movie that you've never discovered. It's moody and evocative.
In fact, mood being the most important element in any Lovecraft story, this evokes the Lovecraft mood as well as anything I've ever seen. Remarkable indeed, as the silent movie conceit could have played as simply a gimmick, or worse yet, a gag.
I'll even go so far as to say, if you want or expect a gimmick or a gag, you're in for a big disappointment. This is a serious, very well executed artistic venture. It's not for a modern audience unused to giving itself over to the experience of a movie and examining it on a largely technical basis.
But for those willing to give themselves over, it's damn satisfying... and genuinely suspenseful and creepy, two things that are increasingly rare in movies.
Kudos to all involved for their hard work and creativity.