Thanks guys!
How does the game compare to Lion Rampant or for that matter, to SAGA?
I like it better than SAGA, as I find SAGA a bit gimicky. It certainly is about the same scale of battle (or skirmish) as SAGA, but it moves more fluidly because of the random activation (which is not as harsh as in Sharp Practice - each unit gets to activate each turn). With SAGA, it always takes me ages until I make up my mind and place my dice on the Battle Board. But perhaps I'm just to stupid to remember the Battle Boards and therefore don't know what to do and what to expect

It's quite different from Lion Rampant - Lion Rampant games feel a bit more like big battles to me and the troop types are much more differentiated. I've heard that Dux does have cavalry, but I don't think it's a big part and I'm not sure it would handle large amounts of mounted troops. I don't think a scenario like Agincourt would be possible with Dux. Also, the cards are geared towards a specific period (450AD to 600AD) (although I'm sure it could be converted).
I like both, and for me, it's not an either/or - they both do different things and model different aspects of (early) medieval warfare. It could replace SAGA (my girlfriend likes SAGA, so I'm not sure if it actually will), but not Lion Rampant.