Stellaris is a 4X/Grand Strategy hybrid game made by Paradox Interactive. Its main features include racial customization, civilization customization, and space empire management. Expansion options include colonization, having other civilizations cede planets to you through war, and vassalizing other empires so that they can be integrated peacefully.
The war system uses the Europa Universalis 4 Warscore system, which allows for only a certain amount of demands to be met in one war.
Pro Highly moddable
The fact that Stellaris is so modifiable makes a huge difference to the game. There are so many ways to customize the game to your liking.
If there's a design decision you dislike, you are always capable of changing it yourself (within certain limits), and you can make the game completely different. You can play as a race from your favorite books, anime, whatever, because so many talented people are making mods for the game.
Pro Emergent stories
Oftentimes, if you play the game without min-maxing everything, you will find yourself with interesting stories to tell. While the coded events themselves may lose their touch after a while, the interactions with the various races nearby will often be different. You might find yourself in situations where the whole galaxy is against you. You also might find a race of primitives who you uplift, who will become fast friends with you. It's possible that you might grow attached to them, only for them to suddenly and violently be taken over by that Xenophobic Militarist on your borders.
The reason some stick with this game for so long is because they love these interactions, despite how shallow they may be sometimes. The fact that you can even be a Fanatic Militarist in the game and still have friends afterward is something that some may find particularly fun.
Con Imbalanced gameplay systems
As the game is right now, without mods, the weapons and other ship modules are sorely in need of a rebalancing. Currently, when you hit the end-game, the best way to go is to stack shields, health, and as many Tachyon Lances as you can fit onto as many Battleships as you can muster, then send them in as a blob to murderize every enemy you come across.
Compared to other games of this genre, this makes the rest of the research you did before hand seem quite meaningless. You might find yourself asking, "Why did I bother researching Kinetic Weapons when I'll never use them in the end?"
Similarly, Ethos and Traits are highly imbalanced as they are. Materialists can get huge research bonuses that nobody else could dream of. Spiritualists can get other races with other ethos to bend to their will ridiculously quickly. Militarists are incredibly good at making war, which is currently the only feasible way to meet the win conditions of the game. As for traits, there are many traits that you may find yourself never picking. Intellectual is always good, Very Strong is always good, Repugnant doesn't matter if you don't integrate other species and Sedentary's malus hardly matters. Hopefully we will see these balanced in the future.
Con Lack of vanilla content, currently
Unfortunately, the game is lacking a bit in certain types of content. There is a great number of events in the game, but not enough as it is. With how many events there are, it is likely that you will see many of these events occur over multiple games. For instance, finding Sol or Sanctuary once may be interesting, but finding it every couple games makes it seem less special.
Con Weak sector AI
In the game, you can create "sectors" to manage the planets that you cannot manage yourself due to the "core worlds" limit. However, these sectors are managed by the game's AI, and the AI often times makes foolish choices with how they develop the planets under its care, even if the "respect tile resources" option is checked. What the option should do is ensure that the AI builds a building on a tile that has bonuses for that type of building. It will instead build whatever it feels like, from what many in the community have seen.