DNA (Deoxyribonucleic Acid) is, and will, revolutionsie Family History. This is on two general fronts.
Firstly, Just because the records tell you that 'Jo Bloggs' was your Great-Great Grandfather, how do you know that actually was the case? No matter how distasteful the subject may be, we know that infidelity, informal adoption, and illegitimacy happen and all threaten to create breaks in a family line.
Now from what I can gather, such occurrences were actually relatively rare. I was privileged a few years ago to take part in a DNA study into the origins of my own family, the Robsons. This found that there were actually 2 distinct branches of the family, of which my own branch went back to a common ancestor who lived around 2,000 years ago. Of the 100 people who gave DNA samples, only 4 did not descend from one of these 2 branches. 4 breaks in 2,000 years. Nevertheless, such breaks are possible, and finding a genetic link to a common ancestor provides definite, no-argument confirmation that the genetic line is true.
Secondly, a DNA link can you give a way forward that would otherwise be unobtainable. Suppose that you are looking for a John Smith born in 1790. There are 3 Smith families - A, B, and C - living in the broad area that you are interested in. But none have a son called John. You do a DNA test and find that you are related to 2 people descended from Family C. The very stong inference is then that 'your' John belonged to Family C but for some reason his birth had gone unrecorded.
Now all this is a bit questionable - the DNA link to these 2 people might actually be through a completely different branch of your family. I find it amusing when people say, 'oh yeah, my name is O'Reilly so my family are Irish'. No. Your patrinliear (father-son) ancestry is Irish, but you have very, very many other branches to your family whose origins might be completely different.
However, as more and more people take DNA tests, and as more and more Family Trees are linked in on-line databases, it will be possible to establish a link with absolute certainty.
Most commercial Family History sites test Autosomal DNA (atDNA) which maps our 22 Autosomal Pairs which contain the DNA of all our recent ancestors. The thing to remember about atDNA, however, is that our DNA can only carry a set amount of information, so with each generation, statisically the contribution from each of our ancestors is halved. So atDNA is only considered good for detecting relationships back 5-7 generations.
The Autosomal Pairs are something of a melting pot, where the various elements of our parents' DNA are mixed to create us.This is not done in an ordered fashion, however, so some strands are passed intact to us while others are diminished or even eliminated.This is worth bearing in mid when someone is boasting of being descended from Lord Such-and Such in the 1200s.They might very well be so, but it could also be the case that this ancestor's contribution to their DNA has been so diminished over the years that it has disappeared altogether.
So, should you do a DNA test? In my opinion the benefits outweigh the pit-falls. I think on-line Family History companies have recognised people's concerns and are going to some length to show that your results will be protected and not made available to third party businesses. On the pro side, it's great to find all these new people that you're related to, no matter how distantly. Certainly in my own case I've made contact with people through shared DNA and been given access to family memories, photographs, etc. that I would otherwise have never known about.
One of my Grandparents was illegitimate and the only clue to her father's identity that we had was that she had the middle name 'Hall'; not a family name. An analysis of my DNA results led me to my Great Grandfather's family though, as yet, not to him specifically. Although I suspect my Grandmother knew his identity, my Mother and her siblings had no idea. In time, someone genetically closer to my Great Grandfather will take a DNA test and I'll be able to pin down the exact individual. In the meantime, discovering my Great Great Grandparent's identity has opened up a whole new avenue of research for me.