The LBS has one on the floor that the owner would love us to take for a week's worth of test riding.
Probably because he knows we would be sucked in and succumb to another case of n+1.
The fatties definitely enable us to ride outside when we may not otherwise be able to. They're a bit harder to push around so a great workout. I keep thinking we might actually race them one of these years, but just doesn't seem to into our "bike" or life priorities yet.
So my advice on fat bikes...... if you want to be able to ride in conditions you might not be able to with other bikes, or you want to do winter single track, or you just like riding a bike you can go anywhere in, get one. Otherwise, it might sit in your garage most of time getting lonely :(