I have read that the MtDNA test is the least useful. Not sure of the reasons why ...
If you fancy treating yourself or want someone else to treat you to a nice present Gadget, I'm sure you will have fun with the results even if they don't lead you where you hope.
Thinking about it, there are a lot of DNA threads on rootschat and I don't recall reading that anyone has taken an MtDNA test .... I don't know if lack of testers here are a reflection of lack of testers elsewhere in the world, which may mean you have very few matches, especially if your family lines remained in Wales.

Still, I'm sure you would gain something from the test.
Added: Wasn't it MtDNA that connected that American chap to Richard the third? It must have some uses.
