I have given a lot of thought to the idea of replacing my MacBook Pro with an iPad, and most recently with an iPad Pro. I use my iPad Mini 2 for as many hours during the day as I use my MBP, but primarily for entertainment (videos, books, browsing, gaming). Many of the things that have kept me from going all-in have been addressed: iOS 9 split-screen multitasking (not as flexible as a full windowed environment, but a big step forward), significant improvements to productivity software like Office 2016, Adobe's CC apps, Procreate, etc..., increased memory/storage capacities, and the large screen / full-size keyboard option of the iPad Pro.
However, there remains one fundamental issue for me and that is file management. Apple has gradually made improvements to its cloud-based file storage option; there are still considerable shortcomings. I have been using Pages and Numbers for active documents, and have enjoyed being able to access/edit them on my mac, my ipad, or a web browser. But the documents have to live in the Pages or Numbers subfolders of iCloud Drive. The new standalone iCloud Drive app allows me to browse all of my files, but if I try to open a Pages or Numbers document, it makes a copy into the Pages/Numbers subfolder. Ironically, while MS 'hides' iCloud Drive in a submenu, it does allow you to directly open and edit documents wherever they are without making copies. MS uses Apple's iCloud better than Apple.
Also: as many have pointed out, cloud storage depends on network access. If I find myself in a situation without network access and didn't have the foresight to predownload all of the documents I intend to work on, I am out of luck. Any device I am going to rely on as my *sole* computing device needs to be able to work effortlessly and reliably offline.
In a perfect world, Apple's own apps would be able to open documents anywhere in its own service without making copies, and iCloud Drive would allow you to pick folders to automatically download locally (syncing back to cloud when network access is available).
I wrote a list of my computing activities in order to figure out how realistic switching to an iPad might be. The ones with an asterisk are the ones that are still problematic to achieve solely on an iPad:
Primary Use Cases
Email, attachments
Contacts/calendars/notes/reminders
Web browsing/bookmarks
Passwords/confidential info management
PDF viewing/editing
* Document archive (see my file management comments above)
Word processing and document layouts
Spreadsheets
* Music management/playlists/ripping and burning CDs (ripping/burning are problematic)
Photo management/sharing/albums/printing
Youtube
* iTunes movies/video library (local family video library is problematic due to size)
Digital books
eCommerce (Amazon, etc...)
Secondary Use Cases
* Backups of key data to external hard drive (backing up iPad directly to hard drive without secondary computer)
Photo editing
Sketching/painting
Maps/directions
Gaming (Hearthstone, Carcassonne, etc...)
* Device firmware updates (infrequent, but critical at times -- like updating car nav system or GPS firmware)
Presentations
Tertiary Use Cases
* Software development (remote development is doable with apps like Textastic, but local dev is not)
Terminal (shell)
I remain optimistic that Apple will continue to improve the iPad hardware, OS, and software, but I think there will always be tradeoffs to be considered. Even my MBP has tradeoffs -- can't draw on its screen, lesser battery life, size/weight, etc... Maybe there isn't one *perfect* computing device.
Richard