I’ve used mine 3 times so far.
When I first received my 11 Pro Max, I was scheduled to travel later that day. I had about 4 hours to get the Pro Max set up as if it was the XS Max I had been using. The PM came with the battery at 65% which I wasn’t comfortable with, so I used the 18W charger while doing all of the updates, downloading apps, etc. Plus I wanted to gauge how quickly this charger would work in a pinch. I was at 100% when I left for the airport.
Second time, I didn’t set the phone properly on my wireless charger one night, so it never charged. I was at 48% and didn’t want to start my day that way. So I used the fast charger while I got ready for work. I left with around 85-90%.
Most recent time, flight delays coming home (eventually got cancelled) caused me to sit around the airport over 6 hours longer than expected. I used the charger while I ate dinner at the airport to top off.
I personally find it very handy and love that it is a compact, portable size not much bigger than the 5W charger that was useless to me. My other chargers are either multi-port or wireless meant mainly for desk/nightstand use.