For me, its another bike I can choose to ride (always a good thing) and it is bags and bags of fun. I know that's not very helpful as it is an entirely subjective view so here are some objective points.
Grip. Simply stacks of it. I have an On One Fatty with their own 'floater' tyres. At almost every point where your MTB brain says "here comes a slide" the Floaters just dig in and drive on. Its an amazing feeling when you are getting away with silly cambered stuff and rooty rooty trails that would have your 140 trail bike slip sliding away.
A floaty ride. My hardcore enduro racing mate (29th at the Ae enduro yesterday- he has credentials!) jumped aboard and remarked immediately that it felt like riding on a cushion of air (because you are!) And would buy one if he had the cash.
They are great everywhere* with the notable exception of the new Paved sections on Ilkley moor. Where they suck. But so does everybike.
Anyone who thinks they are crap has no grease left in their soul and needs a service.