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...since anyone can edit almost any article, it is of course possible for biased, out of date, or incorrect information to be posted to Wikipedia.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 Wikipedia's accuracy is improving all the time, because there are so many people reading the articles and monitoring contributions using the Recent Changes page, and using watchlists, so that incorrect information is usually spotted and corrected quickly. 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!
Read more: Researching with Wikipedia