I'm sure the "Castwell knot" works well (I use the same knot in rope and string for other things), but I don't like having to continually cut off the end of my fly line. When I put the looped butt section on, I never have to touch the fly line again!

It is then super easy to attach leaders. Just stick the butt section loop through the leader loop, then pass the tip of the leader through the butt section loop. Pull the leader all the way through and snug the two loops together.

I can see that if you want to change leaders more quickly, the "Castwell knot" might speed things up a bit.