Kerbal Space has many great points, but also a few things to be wary about. For example it's a game that isn't technically finished yet. In fact, "game" probably isn't the correct word. Although the graphics look slightly game like - including the cartoonish Kermins themselves - there are few game elements at the moment. No levels, or missions, or points.
Instead it's current status is as a "sandbox", allowing you to set your own goals without restriction; and there is more than enough included already to keep you occupied. It already includes the ability to design a huge variety of space crafts, and fly any mission you can imagine. Not only planetary landings, but also deep space probes and solar system observations.
By purchasing the game now you will be entitled to all future releases. However, you should probably base your decision on whether or not you think it is value for money now. Unfortunately the Internet it littered with projects that fizzled out. And almost always in on-going developments a group of early-adopters emerge that constantly complain about how things "used to be be better". You might end-up one of them.
Download the demo
The always-welcome, zero-risk option. The demo version of Kerbal Space has the expected restriction of fewer parts to choose from, but it is apparently still possible to land on the Mun (it takes place in it's own unique solar system). To get a taste for the game just build some rockets and launch them. All you need to know is that you use "stack de-couplers" between stages. Shift/Ctrl keys control throttle, the WASDQE keys control flight, and the space key jettisons stages.
Feeling guilty for sending Kermins to a fiery death? Looking for a challenge? Just like the space faring nations of history, you might want to start simple and practice some important space skills first. The Kerbal Space wiki includes a sequence of missions that you can work through - from putting Sputnik into orbit, to orbital docking, to setting foot on the moon. You can complete some of them using the demo, but at some point you might want to consider investing in the full game.
If you managed to follow all of the examples on the wiki, then you're obviously a natural space-farer. Almost everyone else is going to need a bit more guidance. Some of the best instructional videos available are from the YouTube user Scott Manley. There are of course plenty of others, but Scott seems to have the most comprehensive set - and they're a good mix of information and entertainment.
Kerbal Space benefits from a strong modding community. If you're unfamiliar with the term, it just means that people can make their own modifications to the game - such as creating their own rocket parts or scientific sensors. Mods often show what a game is capable of - if the developers had more time to put into it - so you might want to look through the list as part of your decision to buy. For example, if you like your space simulations a little more realistic, then you might be interested in the Apollo-Saturn add-on or planetary cartography.
Kerbal Space is already a fantastic application, and for the foreseeable future looks set to only get better. The animation-style appearance might split opinion, but the freedom to run your own space programme is unique. With a bit of practice you too can soon be stood next to a broken lander on a distant moon.