Xenonauts. It's greatest strength is that it's X-com. It's greatest weakness is that it's X-com.
If you're an older gamer, played the original X-coms, and just wanted them to get a graphics update and maybe some mods on being able to better organise your units and maybe a few updates, look no further. You've found it.
If you don't know what this "X-com" business is about, both X-com and Xenonauts are tactical and strategic games where you defend earth from an alien invasion through a specialised organisation and squad who deals with the aliens.
You'll respond to the aliens by shooting down the UFOs using regular human fighter aircraft, and then landing a squad of soldiers with just plain old human guns near them to clear out the survivors. After that, you'll begin to research better technology. Laser rifles, faster aircraft to take down the UFOs, armour that protects from the alien weaponry, and so on.
You will meet several different kinds of aliens, each with their own strengths and weaknesses, and figure out why they're invading. But most importantly, you're going to stop them. You're going to take all their technology and make it your own, and you will end up with several bases across the world to find, and destroy any intruders who push into your atmosphere.
As for the changes between this and X-com, they are mostly good. You have an airstrike option on alien crash sites so that you don't need to do everything or have them expire when you've been too busy dealing with other sites, though it will provide you with far less rewards than if you send in a squad of soldiers. There are new pieces of equipment, such as the shield. You'll no longer need to micromanage every last clip or laser battery in your arsenal and order the construction of small things like two laser batteries - Just keep track of the ones you bring into combat so you don't run out. There are now more involved air battles, along with air terror sites. And cover, along with range, now actually provides a bonus or penalty to your shots. You don't have to worry about this turning into a chest high wall simulator though, it works out well and you don't have to stay in cover. It's just a suggestion that increases your soldiers' lifespan. There are some others but they are mostly positive. One I'd say is slightly negative is that you can not land on a UFO and blast a hole in the ceiling to enter it as you could in the original X-com, though if you never played that game(And even if you did), you might not even notice that option isn't there.
Mechanics wise, it works out very similar to X-com, with an isometric view and tile based map. I've found some maps had too little vertical space sometimes but often this wasn't a big problem, the controls are pretty good, mainly just using your mouse to command your units. It's turn based, with a world map to place your bases on and do your air battles, and a base overview where you can build out your base by modular rooms.
Interestingly, I've found that the difficulty settings actually changes AI behavior, rather than the lazy way of just giving the AI more armour or damage. I'm not sure if it also gives the aliens more stats or makes it more difficult to get money for the player, but the fact difficulty settings actually improve the AI is something that many AAA games completely fail at.
In the end, it's X-com. It has most of the strengths of X-com, but it also has the flaws of X-com. You may need to restart several times if you're new and you might get discouraged. A random lucky shot could kill off your best soldier and send the nearby rocket launcher guy into a berserk panic state right into your own squad. If you're not used to it, you'll wonder just how long the campaign keeps going and it may turn into a game of 'whack-a-UFO' for you with slow or barely noticable progress. Or you may simply not have a clue what's going on until the aliens are already in a better position than you.
Even then, it's a very good game. I would say it's worth grabbing it at some point if you want to see what the fuss is about, or because you want some more of the X-com from long ago (Without looking at Interceptor or Enforcer for now)