It has become common for developers to release their games as early access in the recent year. The concept was unthinkable only a few years ago; The few games which launched with a “here’s the game, we’ll patch it / complete it later” attitude received tons of flak for doing so. The practice is now pretty widely accepted; Steam's best sellers list is full of early access games, after all.
Minecraft and Kickstarter happened and paved the way. It’s not just about the product; it’s the rush of communal achievement, being part of a project or cause and maybe even influencing its outcome.
Some of those developers are looking for feedback from the community. These projects gives the early adopter a chance to take part in the development of the game. Not every developer needs or even want that feedback however so don't jump in expecting an attentive ear every time. The main goal here is funding and testing the game and there is such a thing as "too many cooks ruining the broth".
So, early access is another way to help fund a project you believe in and which might otherwise not see the light of day.
On the other hand, early access games are by definition incomplete and could remain incomplete indefinitely.
As such, you, as the player get to play the game in its early stages. You might have your fill of the game before version 1.0 even comes out; your impression of the game tinted by its incomplete and potentially buggy state. If that happens, you will be unlikely to want to play the finished product and experience what video game developers have experienced for years: the inability to enjoy the game to which you contributed.
The sense of community which also makes those games so appealing can turn sour:
The developers could turn out to be less receptive than you'd like.
They might make a decision you disagree with.
They could even encounter difficulties resulting in delays or even project cancellation.
Exploiting the system
Along the well-meaning projects, other early access games occasionally are only there to make a quick buck. A few high-profile projects had very public failures so it can be hard to distinguish a game headed for failure from the next Minecraft since, so far, they’re few and far between.
So should you buy early access games?
Early access games are risky business for developers and supporters alike. The more prudent approach definitely is to wait for the game to get our of early access and be officially released.
However, if you believe strongly in a game in development and want to get involved, at least financially, then by all means…
Here are a few interesting articles on the topic: