Here is a collection of videos I found showing various drone platforms that are able to fly despite a motor failure, one thing that is not exactly possible when flying a conventional quadcopter configuration.
This first video features the 3DR Y6 frame that has a simulated motor failure, due to the coaxial motor layout on this frame a motor failure has almost no impact (other than a slight reduction in thrust) In general coaxial motor configuration multirotors are the best type to use if you plan on carrying expensive gear.
This next video shows a hexacopter frame powered by DJI Naza that is flying just fine with a failed motor
This video shows a X8 Octocopter flying with a borken prop, this time its a basic frame that is using the KK2 flight controller, skip to 5:20 to see the broken prop and the X8 flying with one less prop.
And here is another X8 video powered by the Multiwii flight controller having a mid air motor failure, strat watching a 1:35 to see it. This is quite a cool video in teh sense that the actual motor failure was captured during flight, so you can see how the octocopter sinks slightly due to the sudden loss of thrust.
So as you can see having a coaxial motor configuration is great, but also the regular hexacopter orientation is also fine in the event of a motor failure. As a general rule of thumb, the more motors you have, the less likely you are to crash due to a motor failure, but having extra motors also adds a bunch of complexity , either way here is a fun video showing what happens to a quadcopter when a motor fails (again skip to around 3:50 to see the crash)
But some researchers are working on some software algorithms to deal with a quadcopter motor failure as shown in this video, so hopefuly we will get this added to the arducopter firmware!