Just saw it today. I want to say I really liked it, but I'm not quite there yet. Although, when I have a chance to think about it a bit more I may well be.
What uniquivocaly impressed me was the cinematography. Stunning most the time and frequently uncomfortably realistic. Immediately I felt like I was at the front rather than just watching a movie.
Once I accepted the fact that it was all about the camera work it became quite the visual experience. In that regard there wasn't really anything new to the genre. Trenches, bodies, tunnels, hospital tents, moonscape for no-mans' land, we've all see before. It's just that 1917 does it really, really well! I suspect that if one turned off the sound you wouldn't have felt like you missed anything important. Although the theatre I was at had the sound turned up so high I was afraid I'd come out with a nose bleed.
So, at a minimum the experience is well worth whatever you pay for it. I guess that means I did like it.