The columns in my diagram show the amount of revenue you can expect from each segment. Head terms are the most profitable because these people are already educated about your brand, know what they are looking for and where they are going to buy it from. The Mid-Tail keywords have the least amount of sales because these people are not sure what they want or where to buy from. They are in browsing mode. The Tail has less over all traffic from each keyword but in aggregate it is a very profitable.
Before trying to tackle Mid-Tail keywords, first focus on maximizing impression share of your brand keywords in the head segment. With all other non-brand keywords, build Tail campaigns with tightly themed ad groups and relevant text ads first. If you try to go after the Mid-Tail section of non-brand keywords before working on Head and Tail, you will end up with high CPAs and low conversion rate. You’ll be doing a good job of educating potential new customers down the road with Mid-tail, but based on their search query you can tell they just aren’t in buying mode.