I don't really get what all the negative reviews are about. It's not the best movie ever or that I've seen this year, but I still had a good time watching it. The story was interesting, it had some scary parts, some humorous parts, lots of tension, was well-acted, well-shot, and the effects (both practical and CGI) were well-done. Also, the production design and creature design was great.
A movie doesn't have to be restricted to either being "the best" or "horrible." The theater I saw it in was packed, the audience was responsive and seemed to be loving it, and I myself enjoyed it.
I didn't really play the games, but I was definitely engaged in the lore of some of the MatPat Game Theory videos I saw. And I could tell from audience reactions that there were some cool things in the movie if you knew the games. That's not a negative, to be clear. It's always really cool when filmmakers put things in movies that true fans would catch and geek-out about.
As far as being a viewer who didn't really play the games, I enjoyed the movie. And while I think knowing the games/lore would have definitely enhanced my enjoyment and enthusiasm, I wouldn't say it lacks as a movie or that the games are "required viewing" for those who haven't played them. Everything's explained pretty well.
As far as the horror aspect goes, it is PG-13; I saw some reviews complaining about the lack of gore... It's PG-13. C'mon guys. But, that being said, I thought they still did a good job with the horror elements that were present.
I also really liked the trio of main characters. I thought Josh Hutcherson, Piper Rubio, and Elizabeth Lail all did a good job. Also, I'm glad Josh Hutcherson is back. I hope he'll continue to be in more movies.