Apart from a couple of bad CG shots and a couple of details in the last episode, the first season of Stranger Things borders on perfection. I won't include any spoilers, but if you like films and books like Carrie, Poltergeist, The Goonies, or The Thing, you will probably enjoy this. A lot. Even more so if you grew up in the 1980s.

The second season is slightly worse, less atmospheric, less mysterious, more focused on action sequences, but definitely still worth watching. Characters still behave more or less in character, with a couple of exceptions. Visual effects are a mixed bag, with the new "monster" looking great, but the ones inherited from season 1 still looking fake and badly animated.

The third season, sadly, is a complete mess (both in terms of the overall plot and the behaviour of some characters making no sense). It's still entertaining, in a sillier and more over-the-top kind of way, but it's really closer to Buffy the Vampire Slayer than to the excellent first series. The visual effects are excellent.

On a scale from 1 to 10, I would rate the first three seasons 10, 8, and 6, respectively. Hopefully season 4 won't be a 4.