I use them mostly for gaming. I've found phone screens to be too small, both from a visual perspective, as well as an interface perspective. I couldn't imagine trying to use them for productivity though. Can you get BT keyboards (or use the on-screen keyboard)? Sure, but both typically sacrifice something (appearance, feel, layout) to remain portable. Even laptop keyboards don't feel right compared to a desktop keyboard.
Having said that, as others have suggested, they do have their limited uses. Watching videos, browsing the web and performing basic tasks (sending an e-mail, updating a spreadsheet, making minimal edits to a document) all fit within reasonable use for them. I also think as their prices come down they will be a basic addition for most people for just those very reasons: performs all of the tasks they can with their phone, but on a significantly larger and easier to see/interact with screen. I know the Galaxy Tab S2 is a natural complement to folks that use the S6 or S7: you can take calls/texts received by your phone on the tablet. I wouldn't be surprised to see similar functionality on iPads for folks with iPhones.