I use the rear rack on my Brompton all the time, Mostly, though, I use it to roll the bike around a store or through a bus or train station.
I've learned that a light hold on the left grip lets it roll smoothly behind me. To turn to the left, I just pull left slightly. To turn right, I lightly push down on the grip to release the weight off the rear easy wheels, and the bicycle easily moves to the right.
And the other day I used the rack to carry cargo for the first time! I had riden to the local office supply store for printer cartridges, and realized that I also needed ketchup. The printer cartridges fit easily in my seat bag. The extra large bottle of ketchup from Aldi's would not fit.
And it strapped right on to the rack.
I have never regretted getting the racked version of my Brompton. I know it makes the bicycle a little heavier, and it adds so much flexibility. And I have big plans to use the rack for bicycle touring. My intent is to carry a backpack attached to the back of the seat resting on the rack a la Russ and Laura.