It’s Dwarf Fortress in space.
If you like Dwarf Fortress you’ll probably like RimWorld. The idea is mostly the same: you have a few colonists with some basic supplies who have to build a town, interact with traders, deal with raids (from other settlers rather than goblins), and so forth. There is a victory condition but it’s more of a guideline for people who need it than it is an actual core part of the game.
The graphics are very charming, and other than the interface (which is noticeably more barebones/ugly) do their job quite well. It would be nice if there was a visual distinction when, for example, your colonist has a bionic leg, but this is a pipe dream and probably won’t realistically happen for a long time at least. The sounds are pretty basic and get the job done, but you’ll probably want to put on some music while you play.
The interface is better than DF and a bit easier to learn, but is still very bad. I enjoyed the game regardless, but the terrible UI is one of the parts of this game which needs the most work. There isn’t any one thing about it which is particular broken: the whole thing is just poorly organized and doesn’t reflect the way you actually want to use it.
The game is much more small-scale and slow-paced than DF. DF has you very quickly working up to 50+ dwarves, whereas RimWorld is intended to cap out around 10 or 15: this is explicitly baked into the game as you will see much fewer opportunities to get more colonists in RimWorld than you do in DF. This has some unfortunate consequences for the game: in DF the way you get colonists (migrant waves) is largely independent from ways you interact with the world (traders, goblin raids, etc), so if for some reason DF wanted to limit the number of dwarves to 20 it could just stop sending migrant waves. However, since RimWorld ties these different events closer together, a lot of fun events like raids basically stop happening or are replaced with different versions (such as mechanoid raids) to prevent you gaining colonists. Trading is also way too slow: in my current colony I buy all the steel from every trader with a mod that increases the number of traders that show up and I still don’t have anywhere near enough–not to build an all-steel base or anything, but just to build air vents and the like where they need to go. This would make sense if I was on some kind of deadline to get these things done by, but I’m really not: it’s just an annoying hindrance. Rant about problems with the population cap and the pace of the game aside, it keeps you more invested in your individual characters than DF does: I remember my talker character who got her jaw blown off in a raid and then replaced with the jaw of one of the raiders, whereas I rarely remember individual Urists from DF.
The game has a lot of work to be done: most noticeably, it can get very laggy in the late game, which (if I may project) is probably due to a single developer without too much experience writing fast code and with feature deadlines. It would be nice to see some effort being put into it running faster: I run it on a fairly good gaming machine and it starts getting very choppy at 15 colonists on the default map size. People who are willing to deal with much more lag than I am say it gets almost unplayable (i.e. single digit fps) once you go above 30 or 40 colonists. There is an inevitable point at which lag will take over any colony management game, but DF manages to do just fine with many more dwarves navigating a much more complicated base, since one of the features noticeably lacking (but without too much impact on your enjoyment of the game) is a third dimension.
There’s a great modding scene which provides all sorts of things, from nice visual blocks (ceiling lamps) to gameplay tweaks (richer and more frequent traders) to custom races and other much more complicated things.
All in all, despite its very clear flaws, RimWorld is an entertaining game which will keep you busy for quite a while: even though I’m currently frustrated with the game due to the lag death of my latest and most promising colony, I still clocked almost 50 hours in my first two weeks with the game, which is about all the time I had available given work and other commitments. You’ll almost certainly get the value of your money back, and for all its technical problems it still feels like one of the more complete early access games I’ve played.