I've used pen-based computers forever, all the way back to a Gateway notebook with touch screen. The screen flipped to the back of the case, essentially creating a tablet (that was 1-¾" thick and weighed 7 pounds!). It ran Windows 98 with some custom Gateway software to make the touch stuff work. It was awful, but I loved having digitizer-accurate pen input for sketching, taking notes, and marking up pictures I would take on job sites.
If you are going to use one machine in life, the Surface Pro 3 is probably the one to get. It's small enough, fast enough, and light enough for just about any regular task (3D CAD notwithstanding, which is why I can't use one.)
That being said, it's not the smallest, fastest, or lightest. It's also very awkward. I've used a Surface 2 on a plane, and it's darn near impossible to use as a true "lap"top. The keyboard hinge and kickstand do not work unless it's on a flat surface. And with all the guts and battery of the system behind the screen's glass, the whole thing is very top-heavy and tippy. They've apparently fixed the hinge issue in the SP3, but it still seems unsteady and top-heavy on a lap.
I prefer my 3 device setup, but I understand it's not for everyone. I keep very little info on my actual device. Everything is in DropBox or Google Drive. I use an 11" MacBook Air, an iPad mini with pressure-sensitive bluetooth stylus, and my iMac at home. It's admittedly a bit of a chore to keep everything in sync, but it's nice when I'm on the train and want to read an ebook to not pull out a 12" half-a-laptop. It's also nice when I'm at home to draw CAD looking at a 27" screen. Any all-in-one anything seems like a compromise to me.
It's like those tools where you can buy one motor and attach a drill head, a jigsaw head, or a sanding head. Sure, it's convenient to have all those attachments in your bag, but none of them really work as well as the real tool.