We don't normally get that excited about an iPhone case, especially a large, heavy one. But the FLIR One isn't really a case; it's an infrared camera that uses your iPhone 5/5S as a screen.
Most things emit more infrared radiation the hotter they are, so the FLIR One can see heat. It can see if your house is poorly insulated, it can see through smoke and in total darkness, it can tell you exactly how hot your porridge is, just by looking at it.
This opens up some interesting, if limited, creative options for photography, but mainly the FLIR One is about practical applications: it can see through walls to a certain extent, so you can use it to avoid drilling through your water pipes, or to see where heat is escaping from your house.
The man who demonstrated the FLIR One to us at MWC also suggested that a woman driving into a dark car park at night could use it to check for potential assailants. Unless they've coated themselves in mud, like Arnie does in Predator.
If your job involves needing to know the surface temperature of things, this could be seriously handy. If it doesn't, it's probably good for about two days of Centigrade-based fun. You can preorder one from the FLIR website.