I'm not gonna read any of the above responses in order to keep my opinion unbiased or rather untainted. It's obviously my own opinion, bias or not.
Stellaris is *absolutely* worth the $12 and in addition I'd say Utopia and Leviathan packs are also worth it. Only buy the cosmetic stuff if you wanna support Paradox and like the art, otherwise it's in no way needed for full enjoyment of the game.
I've spent over 270 hours on Stellaris so far, and I'm absolutely far from done. I've played a lot of different empires, civics and ethics and while the core gameplay is the same, the way you approach every new game is completely different depending on your picks. Sure, you can sit down and minmax to your hearts content, but that's not what's fun for me.
The good stuff:
The game has good graphics, great atmosphere and a stellar (sorry) soundtrack. It now provides something to do throughout the entire game, aside from very very late game when it's obvious you are winning, but you still have to nibble at the edges of an enemy empire to actually trigger the win-condition.
End game crisises I'd say are still too easy for an experienced player, but they allow for absolutely destroying the midgame stalemate that can happen when the entire galaxy is colonized and all that is left to do is diplomacy and war. Throw a crisis in there and the entire map falls apart. Great fun.
Combat is more balanced now than it was on release. You can't just spam corvettes throughout the entire game anymore (although they still absolutely dominate the early game).
TONS of lore and events that keep you entertained.
Many goals that show up on the map and assorted event chains coupled with them (Enigmatic Fortress etc).
Warfare has more goals to pick from and it feels like war have more purpose now than it used to. Especially with really powerful strategic resources and "gold mine" planets/systems. There are far more unique systems (such as "Sanctuary) and if you own Utopia, you have several end game goals (the three ascension paths and megastructures).
So, on to the problems:
Early game consists of one thing: Build more corvettes. In order to not get instantly conquered you need to have a fleet cap of corvettes. That's just a matter of fact. Unless you have a full fleet cap of corvettes when you face your first xenophobic/militarist neighbour, you *will* get wardecced and conquered.
If you want to play aggressive early game, you need 2-3 times your fleet cap in corvettes to be competitive. The AI regurlarly overcaps it fleets, even for defense, so you need to have even more ships than they do.
Playing tall is possible, but subpar in the early game and relies a little too much on RNG in the starting position. Wide is still king.
Early tech is 100% useless when it comes to warfare. Until you need either loads of Point Defense (missile neighbour) or you need to go up to Cruisers (late midgame) there is no point whatsoever to waste your research on weapons and defenses. Baseline corvettes are infinitely superior to any kind of vehicle when it comes to DPS/Minerals and upkeep cost. Adding even T2 weapons to a Corvette more than doubles its price and maintenance cost for a 10% increase in DPS. This only gets worse the further up the tech tree you go. So early game combat is dull as balls.
The lack of tactics settings and wing assignment in combat makes for even less interesting combat.
Happiness bonuses are OP as crap. Playing a Spiritualist empire allows you to reach 100% happiness way too easily and then snowball into oblivion with your +25% resource bonus.
Factions are still lacklustre, but far more interesting than before.
Empires are still not very unique even though you can pick different ethics and civics. I'd still like to see gas giant dwellers and see terraforming be far more interesting. Like in Space Empires where you picked at planet type AND an atmosphere type for your race and that would determine what planets you could colonize.
Am I forgetting anything? I don't think so. Anyhow, most of the problems I've listed above are problems that the devs are acutely aware of (as stated in their latest dev corner video) and they also discuss ideas they're tossing around to fix, or work around several of these issues. They have a combat overhaul in the making (because the current system is basically a mishmash of several "just prior to release" systems and therefor doesn't work well), they're planning to completely rework how one holds and conquers space, they're considering FTL changes to make the map more progressive as the game goes on, far more unique systems with resources, tech and events.
I'm having a lot of fun and while there are problems they're not bad enough to keep me from continuing to play. I'm mostly trying to wiggle as much fun out of the game I can *right now* so I can start over and enjoy it all over again when the big changes happen.