I do not have one but my local store did have some out for display (and surpirsingly not tethered down to anything). I tested one out and it feels a lot like writing with an actual pen. I would say they made a little much of the whole pressure sensititvity (at least test driving it with the adobe apps that were loaded on the display ipad pro). Don't get me wrong it is sensitive to different pressures but not as sensitive as a true pencil. But overall it is very responsive. The pencil itself has some nice weight to it.
I am a big fan of "Pencil" by FiftyThree but the apple pencil definitly has a sharper tip and is more precise (and it is not application specific, outside of FiftyThree's paper app, their pencil is just an expensive stylus.
I am still wondering how people are going to keep from loosing it though. It is the size of a regualr pencil, it has no magnetic anything to keep it attached to your ipad when not in use and it is perfectly round (so ideal for rolling off a table). I am looking forward to when they are going to be widley available.