I have not joined it myself, though I have taken part in a feedback survey for their website. I think it could be very helpful if you are still sorting out your family tree. Obviously you have to be aware that there may be errors in any connections made through them - especially if other people
dont put the truth down
So I would say don't join unless you are going to tell the truth
My main reservation is the mission behind it all. I just don't think that my ancestors would want to be baptized etc. into the LDS church. Perhaps some would not mind but I bet some would turn in their graves at the idea!