I stopped reading anything the instant I saw the early reviews were so negative, but after seeing it yesterday, I have to say I think they're massively overstating their case. Probably a simple case of something that wasn't everything they hoped it would be, so it must be terrible (and it doesn't help that everyone's just coming off of Civil War which was legitimately fantastic).
I would say it's the third or fourth best film in the franchise. It doesn't get anywhere near DoFP or X2, but I liked it about as much as First Class or X1 or Deadpool. It does have some issues, but there's nothing really bad in there - just various minor disappointments, all of which is offset by a really great new cast. Overall, I gave it a b-, but I liked the new characters so much that I honestly think it might've been a better movie if they'd just cut a large chunk of the returning cast out altogether.
At the end of the day, it won't change anyone's mind about the franchise. It makes several of the same mistakes that pretty much all the x-men movies make, but it also has most of the same strengths. The story is simpler and more basic this time around, but still strong and interesting, and there's really great character stuff in there for those characters who actually do get focused on, especially Nightcrawler, Jean and Scott.
So here's the more spoilery collection of my random thoughts about what did and didn't work in the movie:
Apocalypse - His famously controversial outfit worked fine. I still would've preferred the tubes be placed differently, but Isaacs had great presence and you really weren't ever thinking 'oh, but that looks strange'. Character wise, he was not everything he could've been. His famous darwinistic attitude (imo, one of the most interesting things about the character) was barely present. He was mainly just an 'I want to rule the world' megalomaniac. But he was good at it, and he had an incredible practicality that was very refreshing in a hollywood villain - no 'I'm going to scheme and scheme and scheme', just 'I need to do this, this is happening right now'. Also, it was disappointing that the growing scenes were exclusively part of the psychic battle. They could have and should have been included in the final fight in the real world. It would've made the scene much more interesting visually - as it was, Apocalypse was mainly just standing there getting hit with various thing.
Magneto - Had great scenes, but ultimately felt kind of shoehorned in. Fassbender is so strong that it still works fine, but i really hope they give him a movie off next time and let someone run an evil scheme that doesn't involve magneto.
Mystique - Her story was mildly interesting and fit with the rest of the movie (and DoFP) really well, but the changing back to human form part was so blatant and unsubtle that it was hard not to think of it as 'and this is the story we had to insert because J Law doesn't want to wear make up'. Honestly, I liked her in First Class and her role in DoFP is amazing, but after this one I kind of hope she's done with the X-Men.
Wolverine - Fun cameo, but completely unnecessary and over the top. This was the 'wonder woman youtube' scene of this movie, which is doubly annoying considering that it isn't even setting anything up for the future. Hugh Jackman's leaving anyway. If they'd just left it at him fighting and cut out the pointless and kind of weird Logan/Jean interaction, it would've been fine.
Xavier - I love his arc over this movie that he comes from a place where he thinks peaceful co-existence will happen on its own and eventually comes around to the idea that the X-Men are needed. This movie really is an X-Men origin story, and I really like that about it. On the other hand - the Moira storyline was just dumb. That's the one thing I really actively disliked about this movie. There's no reason for him to have wiped her memory of that time, no particular clear moment of revelation to explain why he changed his mind, and not even a ghost of a reason for her to be so accepting of what he did. Even worse, playing the story for laughs came off more creepy than funny (since what he did was seriously violating).
Havok - Interesting that they made him Scott's brother despite the age difference. Disappointing that they killed him so fast, and without really even establishing a strong relationship between the two (which made Scott's reaction to his death a bit underwhelming).
Beast - Had some good moments, but nothing major. Very solid overall.
Angel - Amusing that they only ever called him Angel, thereby avoiding the weird continuity issues that seemed inevitable. Of course, the only reason they could get away with it is because he was basically a hollow shell. He could've been any character, and would've worked exactly the same. And now he's dead, so that's disappointing (but par for the course with the x-movies).
Caliban - Was a surprise hit for me. I didn't even know that character would be here, but he was really funny. I can't remember anymore exactly what condition he was left in, but I think he was still alive, so I'd love to see more of him.
Storm - Her part was extremely small, but well done. I liked her.
Psylocke - Ditto, although she seemed a little bit petty at times.
Jubilee - The costume was perfect, the actress was great. The part was, as it turned out, exactly the same size as her previous roles in the original movies. I can't really dock the movie for having a cameo that worked perfectly within its own context, but at this point it is getting annoying that they keep going out of their way to 'include' her without ever actually including her.
Quicksilver - Not quite as strong as DoFP, but close. It was great to see him fight Apocalypse (and to see that he isn't basically omnipotent).
Scott - Very strong. I fully believe this Scott will grow into exactly what Cyclops should've been to begin with. He's already a good portion of the way there, even though he just got his powers.
Jean - Far and away the standout of the movie. She's firing on all cylinders and it's awesome. It's pretty clear they're steering straight for the Phoenix saga and based on the success of these two characters in this movie, I think they'll actually do it justice this time.
Nightcrawler - Just as good as the last Nightcrawler (and that's high praise from me, because that was my favorite character in the franchise). Can't wait to see where he goes from here.
The School - It felt weird to be showing off the blackbird and how great an asset it is only to immediately blow everything up right after. Also, the Quicksilver rescue sequence was a bit overdone. Especially the pizza dog.
The Rest of the World - The nuclear sequence was awesome, but the final scenes with the generals really hit you over the head with the fact that the movie hadn't really engaged with the world at large at all. It feels kind of weird that in the biggest, most globally terrifying event of the X-Men franchise, we got essentially no sense of what it was like for all the people standing by and watching. Also, because of this, the movie basically made no use whatsoever of its 80s setting, which was disappointing, since the use of the historical backgrounds was one of the best parts about this new trilogy.
All in all, like I said, a solid, though not fantastic outing. But it did actually get me even more excited for the next movie, especially since the Post Credits scene basically opens the door for them to bring back any character regardless of dumb character deaths.