It's something that's available on select high-end tablets, such as the Microsoft Surface, and various Thinkpads. It's still vastly inferior to a real keyboard, assuming you know how to type, but I do find it more intuitive than tablet-size touchscreen keyboards. Microsoft's recognition software has very few issues detecting my handwriting, despite it being borderline-illegible when I'm not taking great care to write neatly. It's also self-training, so its detection ability improves every time a correction is issued.
Why other high-end tablets, such as the iPad, don't have this is beyond me.
I've tried using a capacitive stylus on other tablets before, but it simply isn't usable. The inability to distinguish the stylus's touch from the palm's makes it an ergonomic nightmare. Also, the resolution tends to be fairly low, meaning only huge writing will work. With a real digitizer, I can write about the size I would in a wide-ruled notebook, and it gets detected exactly the same as much larger writing.