I have two of them, and while they're a good idea, the execution is kind of chinsy. If you take off the back of the door, you'll see how weak the spot welds are. The continuous hinge is held in place with only pop-rivets. A crowbar would get you in there in less than 30 seconds, and a standard screwdriver would take maybe a couple of minutes. You can make it more secure by installing it with the opening side of the door against the wall, limiting the range of motion for any pry device.
Another major weakness of this and other safes is the combination reset button inside. It's easy to wiggle a coat hanger or piece of bent flat bar in there and press the button to reset the code and gain entry. The fix for that is to saw off the threaded top of a plastic soda bottle and epoxy it in there around the button, then screw the cap on. That will prevent the resetting of the combo from outside.
The Fastbox is okay for keeping just one long gun or shotgun for quick access. I wouldn't put anything high dollar in there. A $200 Maverick 88 12-gauge would be perfect.