Most of the design is very elegant, visible in things like having hexagonal markers for hexagonal spaces and cubical ones to place on played cards. The pieces are sturdy. The box isn't subdivided much, which is annoying with a game that has so many pieces, but they do provide plenty of baggies.
The game is based on creating a solar system. I found this so engrossing that I repeatedly made decisions based on what was more interesting (i.e. making a planet more habitable, or if I couldn't move on that, tormenting one of the hostile ones) rather than what would earn the most points. It uses lots of real-life examples from our solar system and others we've studies -- but it's not all hard science, there are things like alien space probes too.
It's a game that relies a lot on strategy. However, it's in a very rare sweet spot that will appeal both to hardcore strategists and people who don't have much head for strategy. There's always something going on, and moves you can make, even if you're not sure about a long-term plan. Which is fine because you keep drawing cards that give you new goals to work toward. For a strategist, it's a multi-phase game with a complex and evolving plan. For a non-strategist it's just great fun to screw around with. (I can totally imagine Kirk and Spock playing this, and driving each other nuts. Spock would plan everything out in advance, and Kirk would scramble all his plans.) In another rare touch, it's a trees-and-forest game: you have to keep one eye on the small details currently unfolding and one eye on the large-scale, long-term stuff. Again, it's playable by people who favor either, allowing them to play together.
Planetarium is complex enough that most people will make mistakes and forget parts of the rules. Yet it has enough fault tolerance that it's still playable despite that. Try not to angst over it. While the basic concepts are easy to learn, and you can definitely set up a game with new players briskly, learning all the rules will take time.
Something I didn't know when I first picked up the box, but learned from researching it before I bought it: You don't play one planet per player. Instead, any player can move any planet and play cards on any planet. It is generally advisable to spread out your plays across several planets.
Something I didn't know until we read the rules tonight: It has a solitaire mode in addition to the multiplayer mode. This makes it one of the best and most useful geek games ever, because it is a hugely popular topic among geeks. The dual mode means they can play together while sociable or play alone if all-peopled-out. If you're ever throwing an introvert party, or a regular party with introverts so you're giving them a retreat of their own, you need to include amusements like books, puzzles, and/or solitaire games. Any dual-focus game gives you maximum bang for buck.
With two people, we actually found it fairly cooperative, as we agreed early on which planets to develop for life and which to destroy with fire and brimstone. Literally: we played asteroid strikes and sulfur pools on one. :D With three or four players, it could get contentious, especially if people set opposing goals on the same planet. You'll also have a lot more competition for the same resources. However, I think it would easily adapt to cooperative play. You could also easily have teams playing different pairs of planets, or people playing individual planets, and score using the cards played on each planet instead of a collated personal score. It's flexible.
Just based on one sample, it looks very replayable. While the board itself is static and the planets always start in the same places, the resource tokens are randomly placed and the cards are extremely diverse. Even experienced and demanding players are unlikely to tire of this game quickly.
If you are a space exploration and/or science fiction nerd, you'll probably love Planetarium.
It's not cheap -- nearly $50, board games in general are well above my impulse threshold now -- but well worth the investment.