Any encyclopedia, even the printed ones, are only as good as the people who write it.
I think the question of Wikipedia's validity as a source changes dramatically depending on subject matter. In general, the more an issue matters to a community, the more accurate it is. For instance, the Free Software community, which I am a part of, gets pretty accurate representation. Similarly, the Wiki articles about well known celebs are often not only pretty accurate but insanely current.
Hotly contested issues also seem to be "accurate" in the sense that (for example
abortion) both sides have a chance to "sanitize" it.
No single source should be deemed "credible" I think, credibility is built by making a blurb of information relevant and supported by other blurbs.