I took a DNA test with Ancestry a couple of years ago now. In my case I did already have a private tree on Ancestry, but I was still hesitant about taking the DNA test. Having eventually taken the plunge, I would offer the following advice.
First of all, don't be put off by the stories you hear about Ancestry trees. Yes, there are some trees on Ancestry which are complete nonsense, but there are also well researched, carefully assembled trees which are a rich resource for your research. Just remember, an Ancestry tree is only as good as it's owner.
Don't worry about the sources. My private tree is carefully sourced, with links and details carefully explained, and with comments detailing my thinking where there may be confusion or ambiguity in the information available. When I took the DNA test, I misunderstood the information and thought I had to have the results linked to a public tree to get any benefit from eg Thrulines (I now know it works fine with a private searchable tree but such is life). Since I wanted to get the maximum benefit from the test, and needed something constructive to do whilst I waited for the results, I set to work creating a public tree, but made a conscious decision NOT to copy all the thousands of sources from the main one. So the public, DNA linked tree has births, deaths and marriages but zero sources (the one exception being I write in the parish and entry numbers for those records which came from Scotland's People). My attitude being if someone dismisses my tree simply because it appears to have no sources, that's their loss.
Related to the above - don't be a Genealogy snob. Just because a tree has some mistakes, just because it has 16 000 + entries, just because it has a mother with 4 children born in the same year, doesn't mean it is all wrong. It might be that the one tree on Ancestry which provides the crucial link to your undiscovered 5x Great grandparents is that tree with multiple obvious mistakes but which also happens to have a copy of a family bible from the 1700s of your non conformist 4x great grandparents.
ALWAYS remember that hints are just that - hints which MIGHT relate to your ancestor or relative. Crucially, also remember that thrulines are also HINTS. Check, double check and even triple check them. Most of the obvious errors you will find on Ancestry are due to people blindly accepting hints without checking - plus a few cases of even quite thorough and careful researchers sometimes being guilty of believing what they want to believe.
Don't fall into the "20 people can't all be wrong" trap - just because several trees all have the same marriage or the same parents for your ancestor does NOT mean they all researched and drew the same conclusion, nor does it mean they are all correct.
Above all, remember that even if your tree is public its still YOUR tree. If you have researched a particular person and are confident in your own mind that they do belong in the tree, go ahead and put them in. No one else has a right to tell you that they shouldn't be there if you think they should.
