Honestly, I've never tried it out, but I don't really like it, not because of what it does (having a "free" once-per-turn Opportunity Attack is pretty neat), but because of how it goes about it. There's no cost or strategic consideration to Marking this way: if you are gonna attack a single creature on your turn, you just mark them. It doesn't cost an action or an attack, it doesn't really provide any new insights or ideas into how a melee duel goes.