Well, I did read the entire thread and must say I was so happy to see DevonCruwys and JaneyCanuck post with information I know to be accurate and enthusiastic. Now I can just add to their comments.
First, I became interested in DNA testing when my late brother was diagnosed with Parkinson's disease. 23andme.com are involved in research projects and as they were associated with a rather large PD foundation, offered free tests to those diagnosed to aid in research. They have offered the same with regard to other diseases. So, he tested and I managed his account for the genealogy benefits. That was the beginning.
Our family has deep British, Welsh, and Ulster Irish roots. The paternal lines have been in the US starting with the Mayflower in the 1600s, but my mother's Irish ancestry was unknown - her family - by the name of McElrea - arrived in 1902 and left no paper records or oral history.
Through DNA testing, we have connected many family lines to our McElrea One Name study - and so far, we are all related. In addition, we discovered very early roots not to Ireland or to Scotland as we expected - but to the Isle of Man. We continue researching to see if we can learn even more about this interesting family.
My husband always thought of himself as 100% English - he was born in Yorkshire. We expected some bits of Norse or Viking, maybe French - but not much. While I don't put a lot of faith in the ethnicity results, they can point one in a different direction at times. Regardless of percentages, he has many, many matches in Ireland and I have yet to find the source. Also, the fact that his YDNA matches are all to a different surname proved that his gggf had taken his mother's name, not his father's. So there is that mystery confirmed and I can stop searching for someone who does not exist!
I did test my own mtDNA, thinking it might help me down the line because my mother's mother was a Fulton, as was her mother, and her husband's mother. I'll let you think about that one.
Suffice to say there are lots of reasons to test and many problems that can be solved. Besides that, I enjoy the dickens out of it, it keeps my brain working, and I've been the moderator for my local genealogical society's DNA interest group for two years, which gives me somewhere to go on a Thursday evening. Testing does not have to be expensive, as there are at least yearly sales from two of the companies - my favorite being FTDNA. I have also shared costs with cousins and just budgeted when I needed to.
I am 68 years old, have no children, and don't give a hoot what is done with my DNA. Hopefully, it might help someone, as I think my brother's did.
Cheers all