There is a very long and heated thread on this board that began with the question of what did I spend my DNA testing money on re Ancestry.com DNA. The thread gets into many different perspectives and looks at the question from a variety of different ways, but the very end, one poster said "we all agree that AncestryDNA is a waste of money." (I think that post has now been deleted) I don't want to start another heated debate, but if any newcomer is reading this message board, I don't want them to have that as their takeaway; indeed, if you read many of the posts over the past 7 or 8 months, you will see different views about the value.
I have found it extremely useful and has helped me build out my family tree (particularly those troublesome female lines). I think everyone has a different perspective, based upon what they are hoping to accomplish and where they start. Another very helpful poster noted that in choosing a company to do the DNA test, figure out what your goals are as each company offers something slightly different. I can see where people on either extreme of experience - many years, fully developed trees or a handful of people in a tree that don't match names -- will have very different experiences. Also depending on whether your family lines arise - those with Colonial America lines (a primary customer for Ancestry.com) are going to find far many matches than those with more recent emigration from non-British Isle lines.
It does take lots of work to find "good" matches that can be documented, but when you do, you can narrow your document searches etc. If you have already filled out your tree 500 years, you may find it less useful; if you haven't gone back more than 3 generations, I don't think you are going to initially have much success, but you may over time.
I want to reiterate this is my experience, which has been excellent. I have many matches, many validated matches and have opened up new lines of research where I was blocked before. One cautionary note for everyone however is this is a major time-sucker...when you get into it, it is great fun but fully exploring those matches can take time!! But it is important to remember "user experiences may differ."