It's strangely addictive (I've put in 20+ hours over a dozen playthroughs at various difficulties), and yet, at the same time, hugely disappointing.
The story I have to tell in my head about the various missions my team of battle-scarred badasses go on, because the actual in-game story is practically non-existent outside a handful of cutscenes, is a plus and a minus. If you've played miniatures games like Warhammer, gameplay is similar, with a similar lack of enforced story from match to match. You have to imagine what is going on. OK, I guess. I can tell myself a story, but I'd really rather have an awesome campaign story with the option for free-play, not just a free-play campaign with a few crappy tacked on cutscenes.
I can't stand the monster closets they use for every alien encounter. They give you these auditory clues that aliens are moving around on the map, doing alien stuff, maybe hunting your team, but they're really not. It's a lie. On the Terror Attack maps, there are a few aliens that wander around killing civilians in the Fog of War to create a sense of urgency, but every other alien encounter on every other map is one of your squad tripping a spawn trigger, a "oh shit" cutscene of the aliens being stumbled upon, and then they get a free move, which usually puts them within evisceration range. If this happens at the end of your turn, your team is about to get manhandled by one or more squads of aliens getting two turns following your one. If they're Chrysalids, your elite, 15 mission veterans with top of the line gear can get 1-shotted.
The obvious way to dominate this mechanic and not constantly lose guys or have a total squad wipe every mission is to simply inch your way forward, constantly on Overwatch, so that when the aliens do jump out of their invisible closet and scurry towards you, you can cut a few down with attacks of opportunity.
There's never the risk of the aliens setting up an actual dynamic ambush as you make your way through the map, because they're invisible and rooted to the ground until you reveal their closet on the map. Similarly, you can't set up ambushes for them except to do what I outline above.
It would be nice, just once, to hear activity occurring in the distance, move towards it stealthily and not just stumble into the same exact make-up of aliens that pop into existence. Similarly, if I just blew the crap out of the first alien squad and practically dropped the building on them with my rockets and grenades, it would be nice if the other (non-revealed) aliens reacted to the insanely loud firefight happening twenty feet away from them by maybe, getting into cover, or moving to flank me or fall back or charge to their buddies' rescue. You know, do something.
The base-building is straight up boardgame mechanics. There is no political game at all. Every country you successfully run a randomized mission in likes you a little bit more, and a few you couldn't help at the same time get more panicked. You research tech based on when you are in the game plus what buildings (chambers) you built, and what you manage to salvage/capture outright each mission.
Once you actually get into the firefights, it's quite fun to have pew-pew lasers fights with the campy aliens, try to flank each other, use terrain and keep your squad (which you can individually customize somewhat in look and name) alive.
Again, it feels and plays like a miniatures tabletop game, with everything abstracted and relying on to-hit tables with dice rolls. Sometimes you can pull a miracle out of a disastrous situation with a lucky reaction shot from a panicked trooper. Alternately, your elite SpecOps alien-stompers can get wiped out in the blink of an eye despite extreme caution, careful route planning and good tactics/gear when that last Chrysalid manages to dodge everything, including the rocket your Heavy just misfired into your Sniper, and guts your medic for a 1-shot kill.
All-in-all, a decent time-waster you can play while watching TV, but will definitely need some help form either DLC or modders to keep lively.
Keep in mind that the "Normal" mode is actually the "I've never played a video game before" mode and the "Classic" mode is essentially a fatalistic grind towards eventual alien dominance of the world. I can't imagine what "Impossible" mode is like. All Chrysalids, maybe? I prefer Classic w/Ironman. Feels more like an alien apocalypse mini-series where the good guys don't really have a chance of winning without some great strings of luck/acts of god.
You should really be playing with Ironman on, regardless of the difficulty setting, so there is only ever 1 save that is automatic after every significant action. Otherwise, you'll just be tempted to reload after every wipe or really bad roll. Pretend you're playing a tabletop game where your friends won't let you mulligan your way through dumb moves and bad rolls, and the tension really ramps up.