Before the iPad, tablet PCs has been around for quite some time. However, their market penetration had been quite low mostly because of the poor GUI design and prohibitively expensive cost. Previously, most tablet PCs is equipped with a version of Windows XP Tablet PC which usually requires you to use a stylus to enter, click or write anything which proved to be rather cumbersome and inefficient. People wanted to touch and swipe at their tablet just like the iPhone and nobody wanna carry around and use a stylus.
And so Apple created the iPad and to give Microsoft some credit, they have introduced Windows 7 with full multi-touch support for tablet PCs. Don't forget the open source Android and Chrome OS too. In my opinion, tablet PCs will be the future of computing where a lot of people will be using them to do their computing on the go.
I have few experience of using tablet PCs. This one from Xplore Technologies from the U.S is huge, bulky, hot and it weighs a ton. This unit that I got the chance to use is pretty much busted up where the battery is in dire need of replacement. Clicking, typing and writing on this tablet is a horrifying experience, especially with the stylus, even after Windows 7 is installed on it. That's why people, we need a multi-touch, super-slim tablet PC with no stylus where you can just use your fingers to do everything.