It can be a good source for starting out in many mathematical fields. Things to look out for:
1. Errors. While the quality is usually pretty good, subtle errors can creep in easily. Further, blatant 'vandalism' , while usually caught quickly, is also possible.
2. Lack of coherent definitions. Each article has its own terminology, which may not match that of other pages.
3. Lack of proofs. You can learn results, but not how to derive them. Wikipedia has only a handful of proofs, less than a single textbook would have.
4. Lack of exercises. Not a problem if you're in a class and have assignments from the professor, but otherwise it's another obstacle to learning.