The best 20 bucks I've ever spent. I've played Orbiter before, and always had an interest in space travel, so when I saw that there was a game about building rockets and maybe not blowing them up in an effort to get into space, I took a thirty-minute look at the demo version and broke out my wallet for the full version.
That wsa two years ago, roughly. Still playing it to this day. My kids think it's fun to watch, and it even teaches a few things about space travel if you're not careful. Replay value for the game is pretty high, and the recent additions of career mode have made it even better. As with most computer games, there are mods that you can tack on to augment the experience, to make it easier, harder, more complicated, more immersive or simply more fun.
Recent news from the developers indicate that the game is nearing feature-complete status and will be going into Beta mode quite soon. As numerous facets of hte game are still barely implemented or rough around the edges, this is welcome news (I'm looking at you, aerodynamic modeling).
It may not be for everyone, though. Some of my friends tinkered with it only briefly, as it was simply not their cup of tea. My advice for anyone who is even remotely interested in it is to try the demo version, watch a few youtube tutorials, and see if it strikes a chord with you.