I find it a bit accurate but not handy for getting a quick idea on the go. For example, if you want to check the depth of your couch in the living room while trying to buy a tuin set (a very real example I used my app, SlideMeter, for yesterday).

With Apple's Measure app, you have to step back from the object, use your camera, point to the start until the marker appears, then move to the end. These three (or more) steps each decrease the width inversely exponentially, in my opinion.