It's really bad for multi-tasking, all tablet UIs are, and that's the main reason I have a huge distaste for Windows 8, it's fine as a touch device OS, other than how ugly it is, but in the hands of businesses and power users, it's really sluggish no matter how much people have claimed to have adapted to it.
My argument is simple, if it's so well designed, it wouldn't need a desktop mode, if it's so well designed, they'd give people options for the older style UI instead of trying to force people to use such a sluggish UI for desktop computing or non-touch laptops.
It is not a well designed OS for no touch environments, and I will never buy a touch screen that's as large as 40-80 inches because my finger prints would be all over the fucking place, not to mention I'd have to be up to it's face all the time, it makes no sense for home computing, the idea is they simply want to monopolize and make more money for themselves, and they are making us, the consumers, suffer for their greed.
For all of you on the side of MS on this one, just think about this: You are the consumer, MS is not giving you their money, their goal is to take as much as possible and they don't care about hurting others, all the innovation and smaller business will suffer, and might in time hurt you in other ways in the long run, yet you are taking their side, which makes no sense for us end users, at all. Unless of course, if you work for MS, then yes, go ahead and take their side, because you are working for them to get paid.