Common Core is absolutely useless. Not because of the "extra work" but because of the topic the extra work covers. It's not "extra work so kids can learn faster", but instead, it's complete busywork; kids aren't taught the most efficient method to solve math problems initially, but instead they have to work through granual and practically meaningless work.
Don't get me wrong, though - there are upsides to this method of teaching, though usually it ends up where the kid is much more confused than informed on said topic.
However, the downside that I am most concerned with is the lack of "meaning" to Common Core. No, I'm not saying that school is useless, but jobs are starting to become much more specialized, and vaguely teaching "math science english history" certainly does not help any child in trying to acheieve later in life. I've known since I was eight years old that I wanted to become a computer programmer - Common Core practically negates anything outside the realm of these subjects. Most kids who want to specialize, or have interests in said jobs usually have to learn on their own or through their parents.
Finally, I would like to note how demeaning Common Core is on more privately owned schools, such as charter or "gifted" schools. The issue with Common Core in these types of schools is that many teaching methods for "gifted" children require much more homework, and Common Core just adds more to that (unless the school eliminates the rest of the homework in the school in order to impliment Common Core). The "testing season" is also a massive waste of time, as most grades have to test for 2-4 weeks testing, whereas, and especially in gifted schools, kids are usually much ahead of the content and see no use.
On the note of personal experience, I go to a "gifted school" and almost everyone is opting out of the state testing due to the real uselessness of it. The testing applications are miserable, and the tests don't protean to anything that kids learn in school. Common Core is an overrall waste of time, money and energy; nobody has any use for Common Core at this point, unless they want schools to "recognize them" (in which the schools likely won't).
tl;dr: common core pisses me off