The common ancestor filter is incredibly useful. It appears to be filling in lineage gaps with a combination of people from multiple other Ancestry trees, so distant cousins match up, even if someone has only a handful of people in their tree. It's not perfect and you need to independently verify the 'filled in' gaps but so far mine are about 80% correct. Some stunning distant cousin matches, plus two that resolve a long standing question as to who someone was who married under an assumed name.
Unfortunately, several results showed a likely error in my tree for a distant grandparent's sister. I could confirm this and I ended up deleting 350 people! The rebuilt part now makes a lot more sense.
On the whole, where the common ancestor results are not correct, it's because of an error in a tree that Ancestry selected to fill a gap but so far, some of these appear to have enough geographical correlation to indicate a match perhaps a generation or two further back
You can also group your matches, so I've created groups with family names and bunged in all the common ancestor matches for each one. I've then been working through them to see how accurate they are. This is not difficult - if a lineage is composed from multiple trees, you can click on each person to see how the tree owner arrived at that result. You can name groups with whatever you like, so no reason why you couldn't create groups not based on names but for various cousin and grandparent levels
Private trees and common ancestors - while you can't view them, you can still click on them to see 'filled in' trees. In these cases, the lineages have lots of blanks marked private but they are still a big help, plus they identify who is worth messaging for more info.
Some issues - if you search by surname, you can't filter the results. I don't like the scrolling list and much prefer page numbers but the group function helps to deal with that.
I think it is a huge improvement - just don't regard common ancestor results as correct without checking them. Just to make it clear, the old system gave matches composed from two trees - yours and the other person's. The new one constructs matches from multiple trees - more to be wrong, of course, but also much that appears to be right.
[edit] Seeing matched trees is not intuitive. First filter by common ancestor. Second, click on the "people number" for one of the results (be adventurous, click on a 5th to 8th cousin with, say, a tree of seven people). Third, in the screen that comes up, there will be a pair of names in the Common Ancestor section. Click the View Relationship link in either one. Fourth, you get a composite tree appear. Each person in that tree has a clickable link that you can follow to see how the match was derived and who created it.
