There's a good list of things you can do with force touch here: https://support.apple.com/en-gb/HT204352
Some personal opinions about force touch:
My favourite thing about it is that the trackpad is automatically more clickable. The clickable surface area is larger than on my previous Mac (13" Air late 2013), and since I turned off right click (opting for two-finger click instead) I am far more causal about clicking things. This was really, really noticeable when going back to my Air briefly while I was giving it some TLC, and finding I couldn't use the top-left corner to click.
About the "force" aspect of it though... the sensation of pressing it down and watching some on-screen element change in size (which is the case for the dictionary lookup feature) is pretty fun. That's one of my favourite things to show people. It's like watching people use an analogue stick for the first time. At first it's confusing and hard to get the balance of, but soon you become quite proficient with it.
Unfortunately support outside of stock OSX applications is quite limited at the moment. The features that are supported in Safari have not yet made their way to Chrome, and features in Quicktime (changing fast forward speed, mainly) are not in VLC. Stuff like that. Having buttons on the screen which act differently depending on how hard you click them could be quite powerful.
With time, however, I think we'll see developers doing more interesting things with this. It could be quite useful in artistic applications for instance, doing exactly what Wacom tablets (and their ilk) have been doing to simulate the pressure of a brush or pen.
Another good use is as a replacement for right click or CTRL+click, i.e. to open a context menu. This is how it works on the dock, and it's quite nice to use too.
One thing I find wrong with it is that all of a sudden, holding onto a draggable element while scrolling with two fingers (e.g., try moving a playlist around in Spotify when you have lots of playlists) appears to not longer be possible. This might be a software issue with whomever is responsible for the software in question, though.
Finally, the myth that the trackpad doesn't move at all is not true. It moves a little, and if you press hard (as one Apple store employee did when I took it in to get looked at last week, insisting that it "wasn't force touch") it will move down noticeably - so don't overdo it.
This, IMO, is the killer feature of the new MBPR.