Kerbal Space Program is, essentially, an enormous sandbox simulation of a fictional solar system, with a highly accurate physics model. It has a steep learning curve, having to do with advanced astrophysics and rocket science, but has a devoted fanbase stretching from children to several members of NASA, including the team behind the Curiosity rover.
KSP is currently in early alpha but already hosts an incredible array of features, from docking space stations to a full-fledged solar system of six (seven?) planets, including a Mars analogue, a huge green gas giant with four visitable moons (One of which has an oxygen atmosphere) and a planet with an extremely dense atmosphere and radioactive seas. The home planet, Kerbin, where all launches take place from, has two moons, the Mun, which is essentially the Moon, and Minmus, a small moon on the outer edge of its sphere of influence with huge, frozen methane ice plains.
The game's solar system works as a sort of tutorial in and of itself; getting into a stable orbit around Kerbin is a huge accomplishment in its own right. From there, going to the Mun teaches about lithobraking, landings and orbital maneuvers, going to Minmus teaches about mid-course corrections and simply going to any other planet requires a huge amount of forethought and planning, from constructing a huge lifter rocket to get your interplanetary mission into orbit to waiting for a proper launch window. All maneuvers are done with no form of autopilot; as such, there is an immense feeling of accomplishment every step of the way.
Kerbal Space Program is available for only ~$20USD for all platforms, with all future updates (The current in-development update including reentry heat and rovers, both of which have been worked in one way or another through the game's huge modding community) included free, and has a recently-updated demo version for no cost. I definitely recommend you at least try it out.
Anyways, if you've made it this far into the wall of text, feel free to discuss KSP below to your heart's content.
It's a nifty simulator, I'll give it that, but it doesn't seem worth the $15 I threw down on it, and now they want even more.
The problem with this payment scheme (the price goes up a little at a time) is by the time they add enough stuff to allow a purchase, the price goes up, which I guess is suppose to incentivise buying without thinking, which should be a crime.
Back to the game. All you seem to do now is fly rockets into space. Sure, you can have pointless satellites, you can land on the moon, but once you've done it once, it's no longer interesting. The most fun I tend to see myself having if pressing ctlr+alt+shift+d, turning on infinity fuel, and flying across Kerbin at stupidly high speeds.
I neglected to mention the space planes because there is a glitch that will randomly break them. Sometimes, your plane will steer off the runway for no reason, so that aspect of the game is a risk to use.
tl;dr: Not as fun as it would seem, don't encourage this shoddy pricing practice.
Well, it's a game about flying rockets into space. If you didn't want to fly rockets into space, then why did you get it? The point is to make your own objectives. The idea of price going up over time is the same as what Minecraft did; you buy it early, and you can get every part of the development process as it goes. Quite the improvement over some games/systems filled with DLC where you have to pay monthly fees on top of buying the $60 game. I'd say it's quite the bang for your buck, honestly.
And if it steers off the runway for no reason, put more control surfaces/struts on it. Things do break IRL from lack of support.