I recently did two, waited until they were on sale. The first one was 23andme. The results surprised me so much, that's why I did another one with Ancestry; not the same exact results, but similar.
The results were not what I was expecting. I thought I would be 99% English with maybe a little roman invader. They say the results for 23andme go back about 500 years and Ancestry way further back, not sure timeline exactly, but 1000(s)
What I learned (I think-it's complicated and a bit beyond me really), is that although you get 50% from your mother 50% from your father,(and thereby all their odd assortment of DNA ancestors) you also get the 1000s of random dna from ancestors, so for example you may inherit more Scandanavian ancestry than a sibling, and a sibling may pick up more of say Irish ancestry DNA.
I was surprised to learn I have 16% Scandinavian, but that is common for British people and probably about the average amount looking back at our island's history. I've done my ancestry paper trail back to 1600s and can only trace English ancestors, yet the 23andme indicates French/German in the 1800s. Still trying to work it out.