I finished it today and what I think is that it's a decent game despite being a poor Deus Ex sequel.
UI is terrible, and the beginning of the game is not much better with a rather silly early dialogues. Ammunition is universal for all guns, despite it making no sense, and you can already get the best melee weapon in the first hub. Worst of all, however, are levels which are so tiny it sometimes takes just stepping a few meters over less than 5 seconds to move to another one. Wait, perhaps that's not the worst thing, but the fact that it is completely irrelevant if you blast everyone - including civillians - to pieces, or try to do a pacifist run. Opposite to Deus Ex which gameplay-wise did not care, but other NPCs made it clear you're a psycho for murdering kids on the street. Here you can shoot a school and one level later nobody cares. Additionally, the game is extremely easy on 'normal', I should've played on the hardest level probably.
Yet, deep inside it still feels like Deus Ex, just stupidified. You still get multiple ways to approach the goal, but instead of multiple meaning many, here it usually means two. The writing improves after the first hub, and despite the story overall being a bit of a mess due to creators trying to merge multiple Deus Ex endings, you still get that DE atmosphere and conspiracy theories. Quests are usually interesting and get better with time, vibe is there, sometimes quite Blade Runner-like - especially with the holo-AI dialogues. The music is pretty good as well, though it misses to be as memorable as the original's. Invisible War manages to be way better than many modern "sci-fi" games, but it was made at the time when sci-fi was in much better condition, combined with the original game being one of the best it let to huge disappointments and a lot of criticism, but the fact is that it's not a bad game, and compared to many modern ones it should earn even more respect.
Compared to other parts of the series it's obviously vastly inferior to Deus Ex, and Mankind Divinded is much better as it learned from the issues of its prequel. Nonetheless, I'd rate it even with Human Revolution although both games have completely different strengths and weaknesses.
In the end I have enjoyed Invisible War aplenty, but everybody prepared me to it being a garbage whereas I've got a decent action-RPG. It is just not on a Deus Ex level, but it is a pleasant game if you forget of what series it's a part of. Ultimately, I do recommend it as a fair and adequate game, especially if you otherwise played the series. It's far from being remarkable, but it's good enough to have fun and spend about 15h. I might not replay it multiple times like the original DE, but I appreciate I gave it an honest try despite so many negative opinions as it turned out to be pleasant.