What's the efficiency of hub gears compared to derailleurs?
A lot depends on the rider and what you mean by efficiency.

It is easy to keep a hub gear running well for years, but an unmaintained derailleur will become inefficient very quickly.  A hub gear allows the chain to be fully enclosed, for all but the most dedicated cyclist; an enclosed chain will be more efficient as it will be cleaner and better oiled.

An enclose chain allows you to arrive at work, with clean cloths and no need to change, this can save lots of time.  Not having to spend time adjusting a derailleur or cleaning your chain also increase the efficiency of your life.
However it takes longer to remove the wheel if you have hub gears and an enclosed chain.

Gear shifting efficiency. This is most important in cities with many traffic lights to stop at. With a derailleur, you must shift before you stop, or you'll be starting in a high gear. With an IGH, you can forget about shifting while you are braking, and just select the right gear for the next acceleration a second before you start pedaling. It's a huge difference for a commuter, irrelevant for a racer. 

Personally I think on a road bike:


Hub gears are more efficient if you wish to use a bike to make lot of short trips as part of your day to day life.
But derailleurs are more efficient if you consider going fast on a bike to be very important and are willing to put the work into the bike that is needed to keep it in a very good state of maintenance.


(I also like hub brakes for the same reason)