PECS

The PECS Canada website states that it is "straightforward", "cost-effective" and "not overly time-consuming to implement". These are all good qualities to have when looking for a communication device for non-verbal students.
The website describes how the system works as "by teaching an individual to give a picture of a desired item to a “communicative partner", who immediately honours the exchange as a request. The system goes on to teach discrimination of pictures and how to put them together in sentences. In the more advanced phases, individuals are taught to answer questions and to comment." Learning how to use PECS is divided up into six stages ranging from learning how to communicate through a picture/reward system up to learning to comment and respond to questions - essentially engage in conversation - all through the use of picture exchange. 
I think this system would certainly be useful for beginning AAC users as well as users who are just beginning to develop their communication skills. The use of a reward system would definitely benefit kids may be unmotivated to learn to communicate.
I did notice that there are a number of apps that have been developed by the same company that are now available. These apps appear to focus on the same type of rewards based training system.  One app "PECS Phase III" is designed to support the original PECS system. Learning to use the original system, and then layering it with the supporting apps would help train AAC users for more technologically advanced ones such as Proloquo2go - which I believe is beneficial for users to learn to do especially as they grow older. Using a more technology based system would benefit users socially as they get older rather than continuing with a picture exchange system.
The following video explains the PECS system in further detail: