I've been running a daily build of Ubuntu 16.04 (which is not yet released) on my Dell Venue 11 Pro 7140 for a few days. In general, it's very stable, except for an occasional pop-up, "Sorry, Ubuntu 16.04 has experienced an internal error", which doesn't have any other ill effects that I can discern.
I had hoped that Ubuntu's sleep behavior would be better than Windows 8.1, but it's just as bad - the only usable sleep mode is hibernate. This tablet was released with broken operating system support for sleep, and now, a year later, it's STILL broken in Windows 8.1 and Linux. Under Windows, my 7140 still burns 9% of its battery per hour when "asleep," even running the latest drivers and updates.
Used with a physical keybard and mouse or touchpad, Ubuntu 16.04 works very well on the 7140. As a pure tablet, without a physical keyboard or pointing device, using just the touch screen for input, it has a lot of problems. I would be happy if I could use just 3 programs: firefox, thunderbird, and vlc. Of the three, vlc works perfectly, firefox is marginally usable (with some contriving), but thunderbird is not usable for me.
One glaring problem with the touch screen in Ubuntu 16.04 is the inability to right-click. "settings / universal access / pointing and clicking / simulated secondary click - ON" is supposed to simulate a right-click with a long finger press, but it works only in some programs, and then only intermittently. The built-in on-screen keyboard, "onboard", can simulate mouse button clicks, but it's cumbersome - it requires several clicks, and the keyboard covers a significant fraction of the screen when active.
The default text editor, gedit, shows what's possible on the touch screen in Ubuntu 16.04 - it implements scrolling, text selection, and right-click, all with 1-finger gestures, and they work consistently and feel good.
With firefox, the "grab and drag" extension gets you 1-finger scrolling, but it only works if you click a non-clickable area of the screen, it breaks text selection, and momentum doesn't work. I make heavy use of tabs in firefox, which requires right-clicking. Unfortunately, the only way to right-click in firefox is with the cumbersome on-screen keyboard.
With thunderbird, there is no reasonable way to scroll through a mailbox with many messages or to scroll through a single large message. The scroll bar controls are tiny, and moving them the tiniest bit jumps too far.
If you do a web search, you find many suggestions for making Ubuntu usable on a touch screen - ginn, synclient, xswipe, touchegg, mtrack, easystroke, etc. You also find many users saying they tried one or another and they didn't work. I've tried a few of these, to no avail, but I don't have time to continue trying every dodgy recommendation I find on the web.
Currently, my most severe pain point is scrolling in thunderbird. If anyone is successfully using thunderbird with a touch screen in Ubuntu, I'd love to hear how you're doing it. Or, if there is an alternative email client that is reliable, usable on a touch screen, and in ubuntu repos, I'd also love to hear about it.
Finally, one last huge problem - the boot environment has no on-screen keyboard, so there's no way to use full disk encryption in Ubuntu without a physical keyboard.
