I used Ancestry, initially to confirm to myself that my Dad was not my bio father. Turns out he is, which makes his treatment of me growing up all the more confusing.
When the results came back, they confirmed some of my paper (and later online) research and linked me up with people in the US.
Later on, DNA has proved that my grandfather and his twin sister were not fathered by their mother's husband, but by a man from the other end of the county and all my close matches now are in this line.
My parents were very close mouthed about a lot of things, but I believe I have a sibling out there somewhere, probably conceived in the 1940s. Time is running out to find them, but I remain hopeful of a match in that line.
For me, it has been frustrating at times, but definitely well worth doing.