First up, the main reasons that it seems this movie is getting ripped to shreds by critics:
1) John Connor dies. As a kid. At the very start of the movie. Doubleyou Tee Eff, James Cameron. What the hell is wrong with you? It's 1998, Judgement Day has been stopped because of T2, John and Sarah are relaxing at a beach bar in Guatemala, drinking mojitos or whatever, and then some random T-800 walks up and shoots John in the chest, point-blank, kills him dead. Mission accomplished, he walks off and renders the whole of T2 just utterly pointless. As so many critics have pointed out, this is exactly what happened with Aliens 3 where they just killed off Newt and Hicks and gave the middle finger to anyone who cared about the previous movie. Total dick move. The stupidest part of this is that it was completely unnecessary from the stand-point of telling this movie's story. But they had to get him out of the picture so they could turn the movie into point 2 below...
2) {deleted by mods}
Nevertheless, as I said, I liked the movie overall. I thought Mackenzie Davis was good as the "enhanced human", Natalia Reyes was serviceable as "the new John Connor", Linda Hamilton was okay - she really didn't do much except act snarky - and Arnold Schwarzenegger played a much better aged T-800 than was portrayed in Terminator: Genisys.
So the movie starts out with an almost-pre-credit-scene where young John Connor is killed in an offhand manner. The de-aging here is great. Arnold looked a bit off - I think because I'm just so familiar with how 80s Arnold looked - but Sarah and John looked spot-on. Then the T-800 walks off and (as is explained later in the movie) he has no purpose now that John is dead and he's not getting any orders from Skynet, so he just goes off and rescues a woman and her kid from an abusive relationship (unspoken: I presume he brutally slaughtered the husband) and shacks up with them because having a family gives him purpose, and apparently killing John made him sad and gave him a conscience. This ... was weird. It really goes against everything we knew about T-800s from previous movies. They can learn things to make them better infiltrators, but would a terminator just go off and become a family-man and start a drapery business when his mission is complete? Maybe, I guess. Seems not something that a killer cyborg would actually do though.
But that's not all - he wanted to give Sarah purpose because he had killed her son, so he started sending her text messages (yes, yes, that sounds creepy but let me finish) .. he started sending her text messages with coordinates of incoming terminators. Something-something about disturbances in the time-something-something meant that he would know in advance when and where they would be arriving. So Sarah packs up her weapons and frags the new terminator when it arrives. She's been doing this for twenty years by the time Grace (the enhanced human) and the teddy-bear terminator arrive.
Teddy-bear terminator is a new kind of terminator which has a metal skeleton and a liquid metal outer coating. Like a chocolate-covered candy bar, except it will kill you. The liquid metal part can form a "human" by itself, in effect making two terminators (the liquid metal one and the metal skeleton one). Why has this kind of terminator not been seen before - indeed, why does Skynet keep sending T-800s back when it has models like the T-1000 and T-X? Weeeeeell.... turns out that John and Sarah did stop Skynet after all. Judgement Day never happened. Instead a totally different evil AI arose, called Legion, and there was a totally different Judgement Day. (And, yes, you can quite legitimately read the "totally different"s in that sentence in a completely sarcastic way).
How does the T-800 still exist if Skynet was never created ? How did anything in the first two movies even happen? Shut up, that's how.
Teddy-bear terminator tracks down Dani's home and copies her father (i.e. kills him, off-screen), then goes to her workplace which is a factory where she is just the best worker there because she is a girl and the hero and better than any of the men. That's just how the real world should be, all right! Grace also arrives at the factory, just in time, and saves Dani... sigh, look, just go watch Terminator 2 again so that I don't have to write out the entire plot of that movie. It's the same. Nothing is different except the sexes of the heroes.
The first thirty or forty minutes of the movie have great action scenes, but they are just a little bit too familiar, like half-remembered action scenes from a better movie, but certainly they are still enjoyable. Where the movie does differ somewhat is that Sarah Connor is reintroduced to save Grace and Dani from the teddy-bear terminator on a bridge. How did she know to go there? I dunno. If she'd got a text about coordinates she should have gone to where they first arrived from the future, not some bridge in the middle of nowhere. She manages to blurt out a bored-sounding "I'll be back", of course, because Terminator, before Grace and Dani steal her truck and take off. Sarah tracks them down again because Grace has crashed hard (she needs constant meds to keep her enhanced body functioning) and they team up. Grace uses, um, magic (?) to find out where the text messages are coming from and reveals that that location is also tattooed on her midriff. Again, just why ever not? Shut up again.
They go to the location which is a cabin in the woods and find, dun dun DUN, the T-800 which killed John Connor! What!? I was so surprised and ... oh, who am I kidding, they had it in the trailer. Idiots.
That's the end of anything different. The rest of the movie continues on as T2, same scenes, same action set-pieces, just that instead of a good T-800, a lady and teenager fighting a liquid metal terminator, now it is a good T-800, a lady, another lady, and a teenager fighting a liquid metal terminator. But at the very end, instead of dropping the bad terminator into molten steel they (weirdly) go the T3 route - taking the power source out of Grace's body and shoving it into the skull of the teddy-bear terminator, killing him, the T-800, and also Grace dies in the process. Why exactly does Grace the enhanced human have some sort of nuclear power cell inside her body? SHUT UP!!