In addition just because 1 great grandparent is from England, 1 from Spain, 1 from Africa, 1 from Germany, 1 from Scandinavia, 1 from Cambodia, 1 from China, and 1 is 100% Native American does not mean you will be 1/8 of each of those ethnicities.
Yes you get 50% of your DNA from your mom and 50% from your dad, but I don't think anyone has ever gotten exactly 25% from each grandparent. It may be a 30/20/15/35 mix or some other combination.
Plus remember just because your Great Grandparents came from a certain region doesn't mean that is where their ancestors were from. Or even if they have lived in the same region for the last 1,000 years most places have had invaders. Sometimes the invaders stayed in the new region, or sometimes they left little souvenirs behind which affects the ethnicity of future generations.
The ethnicity results are fun and interesting, but the real benefit to DNA testing is in the DNA cousin matches. They help verify the paper trail and break down brick walls.
Just be aware you may find out things you weren't aware of. You might find a close relative you weren't aware of, or you might find out someone you thought was a blood relative really isn't related by blood after all.
I don't want to discourage anyone from taking a DNA test, in fact I actively encourage people to do so. I have tested at both 23andMe and at Ancestry, then transferred my DNA to Gedmatch, FamilyTree DNA and My Hertitage. My feeling is the more places my DNA is the more likely I am to connect with all the DNA cousins I can. And the more DNA cousins I connect with the more brick walls that will fall, either for myself or for others.
In addition to my own tests I have paid for 2 family members to test at Ancestry and 5 to test at 23andMe. As you can see I think DNA testing is great. I have verified some of my paper trails, including one that was based on circumstantial evidence at best. And the more people that test the more likely I will be able to verify the rest of the lines. But I think it is important that people make the choice to test based on facts, not because of fancy advertisements.