To be honest, I only use it to beat up on Veronica.
:o That poor girl!
Anyway OP, Ranger Takedown can be useful when fast enemies are up in your face AS you're backing up. If you want knock people down easier I suggest using something like Golden Gloves or Boxing Tape and taking the perk Super Slam (meaning you now have a chance of knocking your enemy down with Melee AND Unarmed weapons, Req: level 8 Strength 6, Melee skill 45).
Also, I wouldn't learn the Khan trick. It doesn't work.