What can DNA really prove?

What can DNA really prove? I've been mulling that over since receiving test results for another Forgey family line. I had read articles about DNA testing before actually becoming involved myself in August. I had a vague idea of how it would work, and what it would tell me. After two months experience working with results I am getting a much more realistic view of DNA testing for family history.
So far we seem to have a match between Roger Forgey representing the Andrew Forgey, TN, line, and Darren Forgy representing the Samuel Forgey, NC, line. Another person representing the John Forgey line in PA had tested earlier and was not a match. We have at least four other family members who are somewhere in the process of testing, and three others who have at least preliminary results (with 25 markers in for Darren Forgy he is one marker off from Roger Forgey = Genetic distance 1).

What DNA has achieved for us so far:

1. It's confirmed that at least two Forgey lines are related which was impossible to do using traditional genealogy research. They apparently share a common ancestor in Ulster Ireland. We know from our research that the common ancestor link occurred before 1720, which is impossible to document with records.
2. It may have eliminated a relationship with the John Forgy line? Although it is possible that a non   paternity event occurred. It is good practice to have more than one person per line test in case of this kind of event.
3. It has confirmed family tradition i.e. that the family was Scots-Irish.
4. The link between the name Forgey and the name Ferguson has been verified.
5. We have found more distant relations (Gilbert Ferguson, Kenneth Ferguson, George Robert Ferguson, John David Ferguson and Wayne Forgie).
6. A link between our Forgeys and Co. Down Ireland is confirmed. Wayne Forgie's family is from Co. Down.

So what can be expected when we get all of the results back. 

• Comparison of complete sets of results will hopefully give us more data about relationships, and a time frame for the most recent shared ancestor
• A more precise Haplo group which will help confirm or refute a possible French origin or confirm the family had lived in Scotland for thousands of year
• Answer the question regarding whether all the early Forgey/Forgys to settle in America were related or at least come close to answering that question

In conclusion, what DNA can do is tell you is whether you are related to someone at some point in time. It can't tell you exactly when. If you match someone else on every marker except 3 or 4 markers, on a 37 marker test, then you can be reasonably sure the common ancestor lived hundreds of years ago. I was thinking yesterday what if all the Forgeys match perfectly and I can't differentiate the family lines? I remembered that is where all of the research I've collected, and others have shared with me comes in. With 11 years of research on my family line and the DNA test results to back up my assumptions I believe I will have a very well documented accurate family tree when all is said and done.