Well, I'm obviously late to the party.
I just finished watching all 3 seasons of it on Netflix. In about a week and a half. I loved it. I found it quite refreshing and it really is one of the better series I've seen over the last decade or so. I'd put it in the top five.
I do however have a couple of gripes as the seasons went on.
1. The 'Upside Down' was just too overused. They need to ditch it for season four. Three seasons is honestly enough and if they did something with it again or the shadow monster I'll just find myself asking "Really?" and probably watch with a half-baked interest.
2. The 80's nostalgia and cultural references became too overwhelming during season four. I had to stop and laugh when that Terminator-style Russian appeared the first time. It's too obvious, too much and they just need to tone it down.
3. Mike. You loved all the actors in season one because of their youthful innocence but Mike has quickly turned into someone you just want to strangle now. I'm not sure if it's just the way his character is written or if it's because the actor himself has just grown up, but I can't stand watching him complain about everything all the time. Some child actors just can't really cut it when they lose what made them appealing as children in the first place. Could the same be with him?
4. I think three seasons really should have been enough. They're all grown up now. They're in their late teens. What is the backdrop of season four going to be? Mike, Will, Dustin, Lucas et all filling out college applications as they once again fight something evil in Hawkins?