I see the question asked alot. Can a fat bike be your only bike?
Last year (2014) I bought a fat bike in February and rode out the rest of the winter on it. Come April I went back to my Cannondale 29er. Only weeks after getting back on the Cannondale, I found a crack in the frame. The warranty process was started but I needed a bike to ride in the mean time. I put what lightweight bits I could from my 29er onto the fatbike and rode it exclusively from then on.
Riding the fat was fun. I did lots of riding and found the fat bike didn't slow me down much at all. It was a different experience. Loads of traction everywhere, cushy ride quality, looks of awe from others. The fat bike is certainly a capable machine, and a quick one if you spend some time working with the shortcomings of the design. But like any wheel size, there are pros and cons. Its really up to the individual what sacrifices and advantages you're willing to take. A fat bike is a fun option to have too, a worthy addition to any cyclists quiver of bikes. But they do move slightly slower, and even though you have 4" of rubber under you its still a rigid bike and has all the disadvantages (and advantages) of any rigid.
The fat bike is able to conquer nasty terrain much easier than its 2" tired cousins. Techy loose climbs are made easy, cornering in loose sand or loam is also an "on-rails" experience. Jumping and fast downhills do suffer though as you tend to bounce alot with the undamped ride of the tires in conjunction with the rigid fork. Floatation is king with these tires, snow and sand is easily ridden.
In reality though the fat bike is just another option in the age of the "wheel size wars". It was the same kind of fun that going to a 29er from a 26er was. Some people ride fat bike exclusively, some ride 29ers, some 650b, etc, etc. Like I said, another option, and options keep cycling interesting and fresh.
That said, I did build myself another 29er for this year. More options mean more fun, right? Personally, after a year on the fat, I've found I like having a quicker 29er in the fleet. I also can't see myself without a fat bike though either.
The Bric...._ mountain biker, road rider, heavyweight gear abuser. Built like a brick sh*thouse. No bike is safe.