Karabiner (and its predecessor) is invaluable for me every day use of my macbook. In general using a touchpad feels like trying to type with only one finger, but with the "multi-touch exension" (I think it's based on touchsense), allows me to redefine my keyboard depending on what's happening on my touchpad.

So, now my left hand is always on the home keys, and if I'm touching the touchpad with one finger, the home keys are my left, right, and middle mouse buttons. If I'm touching it with two fingers, the home keys and nearby keys remap to save, refresh, find, select all, copy, paste, cut, close, and others. If three fingers are touching, then my keys remap to my windows layout and other utility programs which I often call (such as the terminal).

There are many other general remappings which have made my life at a keyboard much faster and easier to use, but the multi-touch extensions are the ones that I would struggle to live without.

As a side note, if you use the product and find it helpful, you should really consider donating to it. It will make you feel like a better human, and encourage continued support.