Is KSP a perfect game? No, of course not. Is it the near-unplayable buggy mess it's sometimes made out to be? Not at all.
KSP is a really sweet balance of hardcore simulation and tongue-in-cheek silliness. The game models dynamics, kinematics, materials stresses, aerodynamics, and orbital mechanics, all in a way that feels realistics yet remains fun to play. On the other hand, your Kerbonauts have two stats, "Courage" and "Stupidity," and their science experiments include the Mystery Goo Experiment and taking barometric readings in space ("The gauge reads zero," the game helpfully informs you, while rewarding you with sweet Science you can use to buy acces to more rocket parts.)
It is just about the diametrical opposite of a casual game, however. After the first two tiny steps towards space -- launching your first vessel and launching one that can get out of the atmosphere -- you will enter a nearly never-ending cycle of trial and error. Getting into orbit is hard. Re-entry is hard. Getting to Mun orbit is hard. Landing on the Mun is hard. Landing on the Mun, taking off, and returning to Kerbin is really hard. And let's not get started on space planes at this point, those things require some serious work.
If you enjoy this kind of thing, you will love, love, love KSP. There will be spikes of rage as your beloved creations flip out of control, get torn up by aerodynamic forces, run out of fuel somewhere far out in the solar system, or fail in many other exciting ways. You'll go back to the drawing board, figure out what went wrong, fix it, try again, and be rewarded with a massive wash of euphoria when your design WORKED: you made it to the Mun and back, sent a rover to Duna, or a probe splashing down in Eve's explodium seas.
Bugs? Yes there are bugs. They are quite minor in the grand scheme of things: orbital rendez-vous calculation tools flickering wonkily, wheels failing when they really shouldn't, landing gear being unreasonably finicky about angles, parts refusing to snap together properly when constructing a vessel, requiring random punching of buttons to get them to do so, that sort of thing.
At this writing (1.1.3) the biggest issue with KSP has to do with spaceplanes: the atmospheric flight controls are just plain bad, which makes the already hard process of designing spaceplanes even harder. Fortunately modders have stepped to the rescue, and Atmospheric Autopilot completely fixes that problem.
If you like space, and building things, and watching things you built blow up, give KSP a whirl. You will almost certainly love it.