They are stupid fun.
I bought a Cooker Maxi on a whim, just for a bit of a laugh. I absolutely love it, hard to describe but it just makes you smile. It also changed my outlook a bit, altered where I found my fun a little. I now seek out steep horrible climbs as they are brilliant fun with the huge traction you get. Rocky single track decents are awesome too. Am I as fast on it as I am on my HT, probably not overall, but I am faster in some places. Am I enjoying riding it more, yes, I think I am. So much so that the cooker is now for sale and I’ve ordered a more posherer fatty. I think I will mostly ride the fat bike over anything else.
If you ride mostly natural stuff then you should try a fatty, it could make a lot of sense once you try it. If you mostly ride trail centres then its probably less of a good idea. But still fun. I’m looking forward to getting to Cannock or FOD to try it out.
Is it a fad, yes, does that make it less ace, no. Is claiming it being a fad and saying your too cool to ride one as bad as buying one because its a cool new fad, probably is. All bikes are ace and they are all fun to ride (Mostly). Riding inappropriate bikes is often more fun than riding an appropriate bike. If your in a race, then maybe your bike should be finely honed to the task in hand, but if your just dicking about then dick about on the bike that makes dicking about the most fun.