Not literally take over a subreddit, takeover advertising campaigns are typically a high-key screen space domination type of advertising. Think of something like a video games news site where the homepage is completely covered in advertising for a new, high budget game. Ads at the top, ads at the bottom, ads in the normally-empty margins, and often a focus on articles about the subject.
How that reflects to Reddit I'd never know, it's likely something that's exclusive to the newer layout that I have no interest in using.
I'm of the opinion that it has a time and a place, but I do agree that it's exceptionally intrusive to the site's normal experience and should be very rare and short-lived. Any more than a day and its runs afoul of the people who just aren't interested.