I believe that, with its rules and guidelines, any Wikipedia article is bound to improve over time. But it can take a while until an article is properly written.
There are many articles which aren't properly sourced. This can be a problem if those involved in the creation and modification of the unsourced material try to implement their own opinion in the article. Another issue; if the source isn't found on internet but in a book, it is harder to verify. And it's also more difficult, if you own the book and use it as a source, to properly validate its authenticity. But since we're still in the early stage of internet, we can expect this problem to be less important as the years go by.We'll most likely see every single book ending up on the web at some point or another.
The huge problem is that there are articles which require very specific knowledge. Sometimes, only an expert can truly give the real facts, and most people on Wikipedia are just ordinary folks. This can lead to having an expert taking almost entire control of an article because others simply can't match his knowledge - which can lead to biased articles. I had such an experience with the Khan dynasty articles. Being one of the few users having an extensive knowledge of Genghis Khan's numerous followers and their own individual exploits, I ended up writing articles which have yet to be verified or modified simply because no one on Wikipedia appears to know as much as me on the subject.
In the long term, though, I can't really see how Wikipedia would see a decline in its quality unless it changes the way it currently operates. It can only grow and get better. So, yes, I trust Wikipedia on most occasions, but it's still good to check your facts elsewhere if you ever encounter a visibly weaker article. Usually, the more sources an article has, the more it will be accurate.