There are a lot of four and five star reviews here, and I do understand why people appreciate the craftsmanship in this film. (The one-take thing is not that original, by the way, and was executed much better in the German film Victoria (2015) which was genuinely shot in real time in one take)
I have to admit I was gripped and never bored. However the characterisation was two dimensional and dealt almost entirely in stereotypes; the distant, uncaring officers, the valiant and chirpy Tommies, the helpless French and the perfidious, dishonourable Germans. Mendes set out to depict the horrors of war but mainly succeeding in producing a propaganda film. He missed a trick here. Here's an idea; could the story not have been told from the German point of view? I'm sure there were examples of bravery, gallantry and sacrifice on both sides. All this has done is reinforce people's prejudices about Germans in WW1.