In my particular case, the SP3 is replacing my laptop, and NOT my iPad Mini. So I view it as a laptop in tablet form-factor. Others obviously view it the reverse.
I think that's how I view the SP3, too. And quibbles with Win8 aside, that's the main reason why the SP3 isn't for me right now -- I don't really have a need for a laptop. I do have a MacBook Air, but it is parked on a desk and I haven't moved it for more than half a year. In fact, I'm thinking of replacing it with an iMac.
If I order my devices by the amount of time I spend on them, then the iPad is number one, hands down. I've developed a system of thumb typing on the iPad in landscape, and while not as fast as typing on a physical keyboard, I'm fast enough that I do quite a bit of typing on my iPad. Most of my posts to macrumors are from my iPad, most of my text messaging is on my iPad, and I often do my most complex translation jobs on my iPad.
The iPhone, I have with me all the time, but I only use it when it's too awkward to take out my iPad. I can't really think of anything that I do that is better on the iPhone than on the iPad. Well, I use the iPhone to listen to music, but only because it fits in my pocket. I don't even use the phone as a phone much, because I'm hard of hearing. The only time I use the phone is to call someone when I need them to respond immediately to my texts. I call them, say "Read my texts!" and hang up.
So if I have to give up any of my devices, I guess the first to go will be my iPhone. Then the next would be my iPad, because there are some things I need to do that still can be only done on traditional PCs. But I'd really hate to be without my iPad.
So what Microsoft has to do to win me over is to forget the desktop, and make the very best tablet possible, one that is a better tablet than the iPad. But as of now, Microsoft is in danger of losing me on the desktop, because Apple keeps on improving the integration between iOS and OSX. So with each new generation of iOS / OSX, there is less incentive to install Windows on my Macs. I'm interested in where Microsoft goes with Win9 and the next generation of Surfaces, but they've got a big job ahead of them to convince me to switch from iOS.