I have PeterM, I originally did the ancestry dna test and decided to cast my net further, I have 1501 matches there and only 1 so far is matches to me on another site .
I like it because I have the chomosome browser and a list of relatives in common, I can see other peoples maternal and paternal haplogroups. There is a tree as such but 23andme try to populate it with my matches which is tricky as far as my family go, but you can add your own family to start off with. I have found so far people are more likely to respond to messages on there. Also apart from the cost of the test there is no subscription costs.