Apple, remember, is the company that swore blind they would never produce a phone bigger than 3.5".
I've just bought a Surface Pro 4. It has allowed me to clear out of my briefcase:
An 8" tablet
An A4 pad
Sundry laptop add-ons
A USB charger (there's one integrated with the SP4 PSU)
It's a very, very nice device, and Microsoft are to be commended for having usefully combined the tablet (for referring to stuff in meetings), the notepad (for jotting stuff down), and the laptop (for proper long-form writing/spreadsheets/etc. on the move).
In fact, if I hadn't just upgraded my desktop, I might also be tempted to buy a docking station and use it for that, too.
About the only thing that will stop it sweeping the market is the price, I think. Fine as a business tool for those who move around a lot, but too expensive for the mass market. It will be very interesting to see how the sales stack up against the iPad Pro in 12 months time.