vital in my mind
the hornady and forster ones are the best and don't cause the earth. they are split with a grub screw so the don't come undone.
the Lee ones dont lock very well they use an o ring and the redding ones use a grub screw onto the threads which I find slip a bit
with lock rings you can set the dies and forget them and will make your ammo more consistent as your dies won't be moving about. everything is set how it should be.