I have been experiencing problems with app updates getting stuck since iOS 8, and still on iOS 9.2, on two different iPhones (iPhone 5 and iPhone 6), and two different iPads (iPad Mini 2 and 4). I have tried restoring, which helped things for a maybe a week, but the issue reappeared. On my phone I have manual app updates, my iPad has automatic app updates selected. I suspect the issue has to do with some corruption in iCloud/AppStore/Backup settings, since this doesn't seem to be tied to any one device or version of iOS.
I have a lot of apps installed (around 420), so I have lots of apps updating daily, and usually 1-3 a day will end up in some state of stuck. The first, and most common state of stuck installation is where an app in the process of updating stops around 2/3 complete as shown by the round progress indicator. At this point, in the AppStore update screen there is sometimes an option to "re-download" the app, which usually results in the app getting stuck again at 2/3 complete. At this point I usually have to delete the app in Settings>General>Storage & iCloud Usage>Manage Storage>delete app, then reinstall from the AppStore.
The second version of stuck app installation that I see is where an app is updating and the round progress indicator gets stuck on 100%; the circle is clear but doesn't finish to an "installed" usable app state. Deleting the app is usually necessary, though sometimes a restart will make the app go away and I can re-install from the AppStore. Both of these app update issues are annoying because it adds up to a lot of wasted time, and lost data. When Spotify got stuck, I lost 2 gigs of downloaded music which I'll have to re-download over time--hope it doesn't happen again!
I'm posting here mostly to add my experience to the group, and hopefully this will be addressed by Apple soon. I am able to deal with this, but I can't imagine how someone less technically savvy would be able to tolerate this. Anyone who is experiencing this should also send feedback to Apple directly: http://www.apple.com/feedback/