Wikipedia is incredibly popular. Especially with students doing research. However, it is not as popular as an acceptable source with many teachers. I know of faculty who will not accept it as a research source. (Which won't stop students from using it.)
The general complaints against Wikipedia stem from the idea that it is wide open for contributors to post inaccurate information.
I won't argue all the points here, especially since the well-respected science journal Nature already did. They recently published an article concluding that ( for articles on science) Wikipedia is about as accurate as the Encyclopedia Britannica.