It is still viable if you know what your doing. Personally I say it's the slasher weapon to use. The trick is to use the length of it, I've surprised many a foe, both mounted and not, with it's length. The sad thing is that you have NO SPEED at all. The 30% speed reduction is one thing, the speed DELAY from when you release the mouse button and till it starts is the worst.
Also since you do less damage from horse and it's not that high you gonna have to hit people more than once, loads if they got medium or above armour. If you manage to nail the head and get decent speed bonus you can oneshot them sometimes though, but don't count on it.
One thing I found out with it is the surprise factor. The wonky animation is weird, for you AND the enemy, so most people don't know when the hitbox hit, and that was also why I didn't use it in the beginning. But later on when I got used to it, it went really well. Make them jump into the spin and get loads of damage, surprise them with the length of the slash and outrange the lolstab. On horse against horse the surprise can be even better! If your a good rider (NOT a Arabian crutch rider, seriosuly that horse just makes you unable to learn the true tricks of horse movement in this game) and manage to ride infront of their them and slash their horses head will net you a nice horsekill. I don't know how many horses I've killed using this method. Also riding alongside them using your reach and going towards/away can also be nice in horseduels. Remember you got the longest weapon on horse (that SLASH) and nobody is used to fighting you.
