Though the degree of effectiveness is dependant on the enemy. Gremlins are completely knocked down for a good second (used to be longer but its been nerfed for fairness =P) while devlites aren't stunned for a long time and attacks quickly anyways. You either have to wait for them to attack, keep spinning around them, use a gun or lure them to be less grouped.

Now whether an enemy is stunned depends on the speed. With swords, the stun tends to occur with the last hit of a combo or charged attacks and since fast weapons have a long combo while slow weapon have a short combo, you can see which is better at knocking down. Only one kind of gun excluding charged attacks can knockdown which is the magnus series. Most bombs knock down unless it doesn't deal much dmg. (Probably, if you don't deal enough dmg which the knockdown hit, the enemy may not be stunned)

Well anyways, you'll probably best off doing a counterattack (Attack after the enemy attacks).