lock-on basically makes it possible for you to do damage with a knife, when you don't use lock-on, you are basically swinging at the air.
any decent player will move backwards, basically fucking over the guy using knife lock-on while he shoots said knifer.
using the knife is very tricky and stupidly rewarding when its pulled off, you normally want to catch people off guard around corners, and use terrain to your advantage.
im no knife pro, but i generally enjoy killing people with the token melee weapon. for example, the gauntlet from quake 3 or the shield gun (im literally better with the shield gun than any other weapon) from ut2k4.