Jason Snell started the ball rolling with this piece for Macworld, arguing:

After six months of using an iPhone 6s, I’m afraid that I’ve completely stopped using 3D Touch, to the point where I forget it’s there. My opinion about how brilliantly implemented this feature is hasn’t changed a bit, but I feel like Apple needs to rethink the meaning of the 3D Touch in iOS 10 for it to be a more useful feature.

Rene Ritchie argues the flip side in this iMore response:

Everyone’s use-case and perception of value are different, of course, but, for me, 3D Touch is an interface accelerator. Unlike iPad, which is roomy enough to support a two-column view, iPhone can only show one. So, for example, to browse through random items in a list, on iPad I could just tap the items one after the other and skim quickly through whatever caught my eye. On iPhone, however, I had to tap, load the item, then tap back to the list. It’s perceptibly much slower.

Peek solves that problem for me in a very real way. I simply press a little harder on an item that may or may not interest me. If it does, I can pop right in, same as before. If not, I can relax, it goes away, and I can peek at whatever item I want next. No back, no forth, just peek, peek, peek. Perceptibly faster.

I think Jason and Rene, together, make the case that 3D Touch has huge potential but needs some thoughtful tweaks.

Here’s one example. When I try to delete an icon on my home screen, trying to press long enough to get the icon to wiggle so I can delete it, I inevitably take a few tries before I get that wiggle. It takes a subtle navigation between long press and peek. The problem is muscle memory. Here’s what I mean.

The best iOS gestures converts easily to muscle memory. As an example, pinching to zoom in and out requires no thought. Neither does swiping up and down to scroll, or side to side to change pages. Want to return to your first page? Press the home button. Or double-tap to bring up the app switcher. All of these gestures required training at first, but were so distinct from other gestures, they converted quickly to muscle memory.

Part of the problem with 3D Touch is that peeks and long presses are very similar to taps. The difference is a subtle change in pressure, but not one in the dynamics of finger movement. This is harder to convert to muscle memory and harder to remember.

Imagine an elevator with a single button, where you pressed softly for one floor, longer for another floor, and press really hard for a third floor. Think you’d end up going to the wrong floor a lot?

I do think 3D Touch is an important iOS mechanic. A single hard press on the Apple Maps icon to reveal a shortcut to mark my location or quickly get directions home in invaluable. This is just going to take time. Time for developers to find and implement the use cases that make it shine, and time for us users to internalize the goodness.