I've used Oathsworn Cavalry in my Waiqar army a couple of times, (in which case I refer to them as my Oath-broken Cavalry). They are...okay. I don't have enough expansions to fill out my Waiqar ranks, so they are a necessary evil for the time being. That said, they do still get some good use. My favorite tactic is to start with the double march shenanigans to flee far from my own skeletons, and then rally-reform to line up a charge on the enemy. Because we start bunched up, the enemy should be aiming their units at my main force, and when the cavalry march away, it sets them up for a flank. Of course, the flanking charge doesn't happen until round 3, so it does slow things down, and may give the opponent time to turn and face the cavalry.
Eh, it's not perfect, but I'm still working on using them. In one game I set them up to take on an opposing Ardus alone, and that didn't end well for the horsemen.