What are the advantages or disadvantages of a mid wing design?
The mid wing is mostly used in combat aircraft and rarely in passenger jets. The reasons are mostly structural.

Aerodynamically, the mid wing is the best option as it is much more streamlined and has less interference drag compared to the high and low wings.

The mid-wing also has neutral roll stability, which is good from the prespective of combat and aerobatic aircraft as it allows for the performance of rapid roll maneuvers with minimum yaw coupling.

The mid wing has certain disadvantages when it comes to structural design as far as passenger and cargo aircraft are concerned- the wing will have to either pass through the fuselage, eating into usable volume or the structure has to be strengthened around the fuselage to carry the loads. This is the main reason this type of wing is rarely found in commercial airliners.
There are some aircraft which do have mid mountedwings, like the Piaggio Avanti, which has the wings passing behind the cabin and canards and the Hansa jat, which solved the problem by sweeping the wings forward and thus passing it through the rear fuselage. These types are rare, however.



Image from hansajet.de
This type of wings are used by a number of combat aircraft, however, for example, the Dassault Rafale.

Image from http://indiandefence.com
For combat aircraft, the adoption of mid wings means that the landing gear have to retract into the fuselage for keeping the length under check. The mid wing allows for the carriage of weapons and fuel tanks in underwing pylons however. Also, the pilot's visibility is not affected during turns as in the case of high wing.