The mathematics part of Wikipedia is an encyclopedia in the narrow sense of the word. It's very useful in general, but not as good as a place to learn math. The problem are:
- the presentation is very dry,
- the content is often very sketchy,
- there are no proofs,
- "trivial" details, "elementary" examples, "easy" exercises are missing.
Long term, Wikipedia is just too small! What will surpass Wikipedia as a source of information is the internet itself. And since Wikipedia discourages outside links, it won't be an integral part of this depository. And who needs a middleman anyway?
My personal problem with Wikipedia is its anonymity. You can't, and sometimes don't want to, take credit for what you write because:
- you are anonymous yourself;
- anonymous morons will edit or simply delete your contributions;
- anonymous bureaucrats will make final decisions.
For alternatives, see Math online.