It is about as credible as one of those sets of encyclopedias that used to be sold door to door. Which is to say, sometimes credible sometimes not depending on who was editing and how much they cared about truth.

If you want to use Wikipedia as a starting point that's one thing, but never rely on it. To give an example as to why not to just use it, I once spent months going back and forth with one particularly stubborn member who kept changing a cathedrals page to include an erroneous story about Oliver Cromwell destroying one of the wings of the cathedral to shore up a local castle. Eventually I gave up on removing the story, even though I offered multiple sources of information showing the story to be false.