Can you define "efficient"? Payload, range, speed, etc?
Here's a quote from "Aircraft Flight" (Barnard & Philpott , 1995)*:
Since both surfaces on a canard produce positive lift, the overall area, total weight and drag can be lower than for the conventional arrangement.
They go on to discuss the arguments for and against the view that canards are 'unstallable'. Also:
The main problems with the canard configuration stem from interference effects between the fore-plane wake and the main wing. In particular, the down wash from the fore-plane tilts the main wing resultant force vector backwards, thus increasing drag. By careful design however, the advantages can be made to outweigh the disadvantages and highly successful designs by Burt Rutan such as the Vari-eze....provoked renewed interest in the concept.
So, it comes back to efficiency for a given mission. The canard design is also "efficient" for fighter aircraft that need to do tight manoeuvring as the canard produces immediate lift (unlike a tailplane which usually produces a down force).
An interesting thing to throw into the mix is that a lot of the canards discussed on this site have pusher engines. D Raymer in his book "Aircraft Design a Conceptual Approach" highlights that having the propeller too close to the trailing edge of the wing will significantly reduce it's efficiency and therefore overall performance will suffer and vibration problems may occur.
*BTW this book is a great introduction to aerodynamics and stability - blows away a lot of myths about aircraft flight.