Both of these so much. I hate when my ADC flashes when I could have devoured him and gotten him away easily. Even after I've saved them from bad trades a few times, they have trouble grasping that I will save them this time as well, and they Flash away. Or they run away from me, staying just out of reach so I can't Devour them. Or they hop out, as soon as they can, and they hop out the wrong direction, back toward the enemy team, and die.
Also, people don't always understand that Devour can be used offensively as well. For example, low HP Riven and I were chasing two low HP enemy team members. When they got near the turret, I Devoured Riven and ran past the turret, spitting her out beyond the turret and right on top of them for a double kill. Tried the same thing another game, and as I passed the turret, he hopped out, back the way we'd come, letting free kills go, and me taking 3 turret hits for nothing.
And for the ult, I've only had 2 games (out of 30) where people even took my ult. Both of those games, it was a thing of beauty. Pop behind the low health runners and clean up. Pop our jungle to dragon for the steal and clean up. Pop our jung behind their adc/supp for a sweet gank. It's good fun when you play with someone who understands your ult. But mostly I am using it to pick off unsuspecting split pushers by myself, or to quickly get to a lane and clear a massive wave off a turret, or to quickly ward somewhere well past our ward line.