It’s brilliant! It is kinda story driven again, you have events that occur in a window that informs you of them. In 99% there is impact of event on your empire. Core of the game (at least for me) is economy, research, colonization and military aspect. Events in Stellaris can have massive impact. Combat is still very about numbers that pop up, but you can design your ships, therefore you can edit their attributes. Classy “rock-paper-scissors” game is goin on when you are designing your ships. Projectile weapons do more damage to shields, lasers do more damage to armor, torpedoes have no greater damage, but they have considerably bigger range, all od this must be somehow powered, so you have to sacrifice some utility slots for generators, so it seems to be better to use armor, instead of shields, right? Nope, armor slow you down and on corvettes it is simply not desired feature. Corvettes are fast, so they can kill larger ships by avoiding their fire, if enemy ships has slowly tracking turrets, but it can lead to “one shot, one kill situation” etc, etc… This leads to old motto "know your enemy and kill him."
Another thingy is hyperdrive that your empire uses, or is allowed in the campaign. There are three different types, that go from the easiest to the hardest to play. Playing on hardest is kinda extreme, but it can be fun.
Also end game crisis and leviathans give you more challenges even in later game.
Stellaris is overall faster paced game, then CKII or HoI4, but if Paradox will release patches, and keep it updated, it will be a good game for long time. I have played one game of CKII for maybe a year or even longer, spending with one ironman campaign more than 100 hours. Currently, Stellaris has much shorter campaigns and somewhat limited map (never conquered whole map in CKII, in Stellaris I did it).
Wow, thats kinda wall of text…
Edit: and i still have things to say!