Plain and simple - If it isn't already updated, and it isn't rooted, you can choose to defer the update a few times, but you won't have any choice but to accept the updated after that.
Wife's phone is not rooted, and updated. She says she notices improved battery life, but your mileage may vary. 4G used to drop out for no reason, that seems to have been fixed. Some bloatware was removed. Little changes in the way it looks. But overall, nothing major in the way of improvements or problems with her phone.