The game has my recommendation. The levels are complex without being overwhelming, the combat is fluid and fun and there is a decent-ish enemy variety (although the game needs to pass the first third to see it) and the AI is better than it may seem at first. Discovering secrets in the levels like an old school game is a blast, and of course the feeling of being in a place where there is not up or down, which is the secret sauce for this type of games, is great. The difficulty progression is also adequate, around level 7-8 as krayzkrok has said the game starts being ‘serious’, needing a decent but not overwhelming skill.
The game also has an adequate duration, around a 8.5 hours campaign, not counting the endless skirmish nor multiplayer.
My only con is that that difficulty has another spike in what I think is the second-to-last level the second alien level, where things start to being too difficult, and you have to abuse too much the quicksave. At that point I wanted to lower the difficulty and finish already… but I couldn’t, there is no way to change the difficulty of a current game. :(