Before I continue, I have to say that The Stanley Parable comes in three editions: a demo, a mod, and a pay-for game. I have played all of them, but I'll focus on the latter. The demo is something entirely silly, and not really related to the others. The demo is constructed in the same thematic vein as the others, but has none of the same content. The game is an expanded version of the mod, and builds on the ideas from it.
In this game, you play as an office worker named Stanley, whose job is to press buttons. There is no obvious reason for doing so; there's no indication that he is in quality assurance for anything. The first thing you notice about the game is the narrator. You can choose to listen to him, or not. Thank goodness that the game is more forgiving than the mod, where you would pretty much die at every turn. The gameplay is primarily driven off the route Stanley takes through the levels, rather than pressing buttons, something that Stanley might not have been trained for.
There are multiple endings, depending on what paths you take. Depending on what you do, the narrator may become hostile, which for me is it's own payoff. I've got to say that the confusion ending is my favorite, because another character of sorts is introduced and becomes trusted, only to grief everyone else. There are endings about what is the narrator's vision of true art, what is happiness, one where the environment falls apart, and one about the broom closet.
I admire the level design. I somehow enjoyed how the levels reconfigure themselves behind you. For instance, in the confusion ending, you are able to walk through any one of 8 doors in a room, and no matter which one you choose, so long as you keep heading in a general direction, you will get back to where you were. Other times, you will be walking around a corner of a hallway, only to find another corner not far ahead. You will go around more than four corners and never get to where you came in, nor your destination. Maybe that's another thing: maybe someone's mapped out the Half-Life games in their entirety, and noticed that we've been running in circles this entire time.
The whole thing is meant as a commentary on video game tropes. I'm not sure what types of games aside from extremely linear and scripted games that it's trying to comment on. Some games have multiple endings, some have sad endings even if you do as you're told, some have no story, some loop around, some aren't linear at all, and for others, linearity isn't the point.
Overall, it's worth what I paid for it. There are several easter eggs, and the humor comes by the loads.