Studies are great, but I like to rely on my own experience and common sense as well. Here's what I see when my son uses devices, and what I've noticed about when I/other parents I know choose to use them:
A tablet or iPhone is endlessly stimulating. Flashing lights, immediate feedback, all way more stimulating (I mean in terms of lighting up those novelty-seeking dopamine transmitters) than any real-world activity. My son behaves almost like an addict if he has regular access to a device of any sort. He literally doesn't want to do anything else, and if he can't have one when he wants one, he also has a hard time playing on his own, and he tends to have more angry and impatient outbursts. His tolerance for boredom drops almost to nothing.
Because of these issues, we limit his use of devices most of the time. Of course there are times when the temptation is too great: no matter how much I try to prep beforehand, devices always get heavy use during events we can't skip (rehearsal dinners, etc), for example. At home, the more distracted my wife and I are, or the more tired we are, the more likely we are to let my son spend some time with a device. What he actually uses the iPad for might not be inherently "bad" and might even be educational in some way, but he's also picking up from us that the way to deal with unusual or uncomfortable situations is to retreat into the most stimulating thing you can find. (Obviously, we could counter this by creating time for him to use devices regularly, but that brings us back to problem 1. He is way overstimulated by them.) So in our case, using devices to get through difficult social moments is basically a way of teaching him to self-tranquilize, which is all the more disturbing since it's with one most ubiquitous and addictive items available.
tl;dr: Read the studies, but don't forget that your kid is a pretty damn important source of evidence.