I was thinking of doing this for my wife as an Xmas pressie. It seems very clearly outlined on the Ancestry site what you might expect, so that is not an issue for me. I won't be expecting anything more than an email with a few percentages covering some fairly broad geographical areas.
I want to do it really for the LOLz as my wife, just like any other American has a very clear idea on here heritage (Sicilian/Nordic/German) based entirely on flimsy evidence, hearsay and a few frankly ridiculous family stories passed down over the years.
Whatever the results, it will be interesting for her.