Excellent cinematography and fast moving plot. But as others have noted way too many flaws and silly little errors. It's an entertaining film but not an accurate historical battle movie like Waterloo, Zulu, or Gettysburg. It's more like a John Wayne western. For starters the entire premise of the film is a bit ridiculous. Why even bother sending 2 men on foot, through dangerous enemy territory, to deliver this extremely urgent message that will save 1600 soldier's lives, when there are British planes flying around. The general could have had a plane fly over to the Devons in 5 minutes and land safely on the nice green field in front of the Devon's lines and deliver the message.
Other flaws: Schofield gets buried under collapsing rocks & bricks and a couple minutes later all he does is wash dust from his eyes and Hey Presto he's good to go, not a scratch on him, no bruised ribs or anything.
After Blake gets killed by the German aviator, Schofield takes the letter from his dead body and the rings from his fingers. What about the compass? What about the magazine from his rifle. Schofield doesn't have need for an extra mag of bullets?
And weren't those Germans terrible marksmen. From the time Schofield enters the ruined French town until he falls in the river, Germans are chasing him and firing countless shots but none can hit him. This is totally John Wayne fantasy stuff, not battle realism.
Finally, after being washed around in a river and over rapids, Schofield emerges soaking wet to deliver the letter to the commander of the Devons. In 1917 the letter would have been hand written by pen & ink. So after being submerged in river water the letter would be totally illegible as the ink would have run.