The lock is especially powerful if you combine it with lists. For the longest time I didn't know about lists because I didn't use FB's chat or any similar features that revealed lists to me. Being able to quickly block just "Family" or "Old Bosses" via lists and locks is one security change FB made that really works for me.

(one big problem, though, is that I don't think there is a way from Facebook mobile to use the lock's functionality.)