It's pretty good, and getting better. But...
...since anyone can edit almost any article, it is of course possible for biased, out of date, or incorrect information to be posted to Wikipedia. Therefore, it is advisable to double-check what you learn in Wikipedia against other sources.
Keep in mind that there are so many people reading the articles and monitoring contributions using the Recent Changes page, and using watchlists, that incorrect information is usually spotted and corrected quickly. Of course, new errors are also introduced all the time, so that the overall accuracy rate isn't really improving. Still, you are encouraged to help by correcting articles, validating content, and providing useful references.
The more accurate Wikipedia becomes, the more it attracts additional contributors. It's a self-reinforcing cycle!