I tried Summon Creatures of the Night in my last game, versus Skaven. My army contained 2 units of 5 dire wolves, a unit of 5 fell bats, and a unit of 2 bat swarms. The bat swarms and one of the wolf darts were out of range of Invocation, but in the first turn the fell bats went from 5 models to 8, whereupon I ran out of models, and in the second turn the closer dire wolf unit went up to 20, whereupon I ran out of models. The large unit of bats threatening his warmachines prompted my opponent to charge them with the unit containing his warlord, which left him in a very poor position and allowed me to blend his general with my vampire. The large unit of dire wolves charged and tarpitted the skaven slaves for the rest of the game. (My opponent did not appreciate it when I pointed out that the skaven slaves were being tarpitted by a cheaper unit, but I couldn't help myself.)
I miscast on three dice in both turns, the second time losing a few magic levels, so from turn 3 on my opponent was able to consistently dispel my Invocation attempts.
One game is a small sample size.
I took the power because I already had the relevant units in the army in a list that relies on the vampire lord for its magic rather than a necromancer, so I figured that in one turn the power would pay for itself.
The biggest problem I had, other than running out of models and miscasting, was fitting the models in. In the future I will need to remember to leave more space around the units in the early game in case they balloon.
I felt like there was a definite psychological element to the game. I think my opponent watched my units suddenly get huge (I also had a unit of zombies that went up to 60-something), and panicked a little. (Although he was right: 8 fell bats flapping around behind his lines after eating his warp lighting cannon would have been really bad for him.) It's not just a matter of watching your opponent's units increase in size; the battlefield role of those units changes, and I think that can throw people off.
Now I just need to buy more dire wolves and fell bats.