I did.

All the time. Small amounts of up to $5.

Anonymous wrote:

NP here. I also find it to be pretty unreliable. Good for getting a basic grasp of some things, but I don't trust it.

Another NP. I did find a mistake once in something I knew about. It's been corrected. I brought it up on the "talk" page, provided primary-source citations, and while I was still trying to get up the nerve to make the change someone with more experience kindly did it for me.

I have donated, too. No single source is ever going to be perfect, but with Wikipedia there are often accessible links, while with a printed book you usually need to go to a major library to check up. Don't ever rely on one single secondary source for anything important.

I did once.