Parts are really good, others less so. The advantage of wikipedia is that experts can jump in, improve articles, and end up producing some really good content. I think that the main article on Anglo-Saxon migrations (something I know a fair bit about) does a pretty good job condensing the current state of scholarly consensus and research. It has problems, but it's as good as many other overviews, and it's very clear that some academics who really understand this field (no idea who!) have put time into making sure the article is good.
But the problem of wikipedia is the same as its advantage: anyone can submit content and improve articles, but because it's completely voluntary, there's no guarantee that good experts will see every page. A lot of people like to raise fears about vandalism ('anyone can write anything!'), but I actually think that's less of a concern that the lack of guaranteed expert attention. Wikipedia's massive staff are really good at spotting vandalism (in fact, some of my students have tried - and failed - to improve articles because the editors didn't think their improvements were good enough), but there's no mechanism to ensure that every topic has been vetted and improved by a knowledgable expert. A lot of articles, especially about history, are direct copies from public domain (ie: oooooold) encyclopedias, or else quick summaries that someone wrote once-upon-a-time, but which have never been reviewed and improved by someone who really knows the historical literature to ensure that they're up-to-date and up to scratch.
If you know enough about history, you can usually spot the good articles. They not only cite sources, but academic sources from recent years, and explain current debates in a way that shows the authors clearly know what they're talking about. Once you read enough history, you can learn to spot these pretty reliably. But - and here's where wikipedia's biggest problem lies - if you don't know a fair bit about history, it's a lot harder to sort the good from the bad.
That's why teachers almost always tell their students to be cautious of wikipedia, and to go to libraries for their information instead. Once you know enough to separate good from bad history, wikipedia's a great tool. But if you aren't a trained academic who's spent several years working on fine tuning your BS/quality meter, you want a source that you can always trust to have been written by an expert, not a source that has a completely random 50/50 expert/outdated mix.
So I would recommend staying away from wikipedia, and - if you are able to - instead go to your nearest public research library and consult the specialized encyclopedias they have. Not encyclopedia Britanica or whatever, but things like the Cambridge Medieval History (books like this exist for almost every bit of the past you're interested in - a university librarian can show you where they are). In these books, everything is written by a leading expert on the field, and you know you can trust their summaries of events and time periods to reflect the latest research.
Wikipedia can be good as a starting point, but it should never be your final source. Some articles can be well written, but articles on controversial subjects (or even those tangentially related to controversial subjects) often change back and forth between varying viewpoints.
Wikipedia articles are only as good as the sources they cite, so looking at the sources in the bibliography can give some insight into the quality of the article.
The best historical sources are generally books published in a academic press, or journal articles in databases like JSTOR.
Hi, there was a thread not long ago on Wikipedia that may be of interest