For me, the Surface Pro would be a laptop replacement that can also be a tablet. I don't own a tablet, because 99% of the time, I don't know what I'd do with one. I have a nice, powerful, light-weight laptop that I use as my primary work computer (attached to an external display and keyboard/mouse) and when I'm e.g. at a conference. I have a smart phone for when I need to read mail or use maps or find stuff on the web when I'm on the go. And I have an e-Ink reader for reading books.
The only time I feel I would like having a tablet is when I'm traveling; in an airplane, or sometimes even at a conference depending on their seating arrangements, a tablet would be more useful. But I'm not going to buy a separate, expensive device for just those scenarios.
The Surface Pro or something similar would be the workhorse laptop that can run Visual Studio and do all the things I need to do at work and can be attached to an external display and just function like a regular PC. Yet at the same time, it would also become a tablet for those situations where that is more convenient without having to own and carry around an extra device.
That is why I look at the iPad or a Windows RT tablet and see a toy that's kind of neat but that I, personally, just don't have a need to own. But when I look at the Surface Pro I see something that, next time I need to replace my laptop, I can get instead to fill both roles at once.
Of course, it remains to be seen whether the Surface Pro can actually work that way. It could be too heavy for a tablet, or too underpowered as a serious laptop replacement. But if not the Surface Pro itself, the idea of a device that can be both really appeals to me and I think that's a niche that Windows 8 can fill that iPads or Android tablets definitely can't.