I completely loved this.
The atmosphere and overall vibe are outstanding, as are all of the main performances. The kids are really, really fun together, and rather than finding the characters lacking, I was actually thought almost all of them - the kids, the teens, and the adults - were fairly well drawn for a genre show. Winona Ryder delivers some career-best type work in this, and David Harbour, somebody I wasn't terribly familiar with before watching, has earned a bigtime new fan. Damn, was he great. I also found the plotting to be fairly compelling, and played into - and also subverted - familiar tropes in ways that I found interesting (just as examples, the mini-triangle between Jonathan, Nancy, and Steve doesn't play out quite like the more pedestrian version of it likely would, and a choice Hopper makes late in the game is more a more nuanced one than you might expect one of our "heroes" to make).
Nitpicks? The "villains" are totally one-note, which is kind of a bummer. I was hoping they would steal even more from E.T. and have them not be villains so much as antagonists, but in this case they pulled more from King's "The Shop" than from Spielberg. I was also kind of unimpressed by some of the creature effects once they take the forefront in the second half, but that's a pretty minor concern, and they try to hide it as best as they can.
Other than that, though, I was thrilled by this, and it hit me right where I live. Hopefully we get a second season (as there are some interesting dangling threads left that could be explored, and I'd love to spend more time with these characters), but if this is all, it's a pretty self-contained and satisfying piece of work.