I have had a few hits with the Theory of Family Relativity on MH and some appeared to be correct.  On the other hand I have had many hits on ThruLines on Ancestry.  Partly it depends on the depth of your posted family tree.  By now I have nearly 10K people in my family tree, including many people at my generation, 4th, 5th... cousins.  The fuller you make your own tree, the more likely you are to get hits.  Both Ancestry and MH have done a good job but Ancestry has many more customers and so more chances to find matches.  But I have to admit that I am having trouble at the moment with Ancestry and ThruLines because I have listed adopted-parents for my husband's grandmother whose parents died when she was 16.  Ancestry has included these adopted parents in Thru-Lines as blood relatives and I can't figure out how to get rid of them.  They came from a seemingly prolific London Sephardic family (though they, themselves, never had children) and so many of their relatives are shown as my husbands relatives at the level of 8 cM, which is meaningless.  
Jocelyn