At the outset I am no expert whatsoever in DNA and can only discuss it from a novice's perspective.

12 of my cousins and I have undertaken our autosomal test through FamilyTree DNA using the Family Finder test. 3 of the men have also had the yDNA test. My partner, and his mother and sister have just submitted their kits this week.
Why did we initially undertake DNA testing?
We were attempting to resolve the question of who was our great grandfather? Sounds simple but sitting behind that question lies a group of people determined to stop others seeking the truth for a range of reasons.
Outcome
We now know conclusively our 2nd great grandfather & are content

. However there are still a group of angry people

Other unknowns.
We then set about trying to locate our 3rd great grandmother origins; she grew up on one of the large estates in England. We know she wasn't a servant but also not recognised openly by the family. She had a guardian established by the family.
We are currently working on that aspect; all data has been uploaded to GEDMATCH to enable comparison across all testing agencies. This is proving very interesting.
The ethnicity aspect [My Origins] was fascinating. For example, two of my full brothers and I had very interesting results. Attached is a clipping for my 2 siblings and I.
As to my siblings & my origins; whilst we are Australian, in most instances that means we are a rather heterogeneous lot.
Of the 16 - 2nd great grandparents:6 are English, 3 Irish, 2 Welsh, 1 Chinese, 1 Danish, 1 German [NB that adds to 14 but 2 were first cousins and 1 is as yet unknown]
As you can see from the clipping one of us has nil or negligible Scandinavian [0%] where the other two have a small amount or a much larger amount being 6% & 24%. The difference in our looks is rather interesting and the one with 24% Scandinavian looks Scandinavian. The clipping tells the story better than I.
The unknowns in our origins are the 3rd great grandmother, who to date may well have some Jewish elements, the Chinese aspect is unknown other than our 2nd great grandfather hailed from Amoy; where his parents came from is unknown and may never be known.
I am also hoping that we will find a link into our Danish ancestors so still working on that one.
Would I recommend DNA testing? Yes - I certainly have learned a lot and find it of great interest even though it takes a lot to develop a whole new field of understanding. However a number of my elderly aunts and uncles would not have their DNA tested at all and seem very resistant to the idea.
One of my siblings is a tad paranoid and worries about the safety of the results and actual DNA material. This sibling is not overly trusting of governments and the enforcement agencies even though this siblings has no reason to be concerned and as a result is worried these agencies would be able to access the material..
As to the expense: its maybe 20 cups of coffee at the most.
DNA testing has been of great benefit especially as we have 12 cousins tested - this has helped enormously to identify definate maternal or paternal matches as an example. Both my parents are deceased so we are unable to use that as a starting point. My cousin-in-law is currently looking at particular DNA segments and aligning those to lines and this is proving very beneficial.
So yes I recommend it but it really is personal preference.
Westy