Actually pretty good!
I'm one of those rabid gen-1-only fans, but there were a whole bunch of gen 1 Pokémon around, so it wasn't a big deal. The effects were great, there were some genuinely eerie moments, the technology made it feel very scifi (but it was normalised - there was no boring explaining-the-basics scene), and the plot was good too! Pretty solid characters, motivations, and stakes, which is refreshing. Almost walked out (to watch another day) because the cinema had messed up (the first part of the film was horribly blurry), but it was good enough that I had trouble walking away from it even when it was a literal eyesore, and then that got fixed and it was a joy to watch.
As someone who went mostly out of morbid curiosity plus an urgent need to see more of the Bulbasaurs from the trailer, I was very pleasantly surprised!
(Also: FINALLY a film that releases in the UK on the same day as the US rather than months later! Only a week after its Japan release, too.)