Similar as in they are both fantasy games. A few mechanics are similar, but the games ultimately play pretty differently. DD is more action oriented with special attacks, combos, scaling larger enemies etc.
I am currently playing NG3 Razor's Edge (among other games) and I gotta say I really like the game. The graphics are great, the game is challenging, the cut-scenes are cool, that ninja chick looks good... I mean that seems pretty cool to me.
Currently I'm stuck on the *SPOILER*
The Dinosaur at the moment lol It makes me laugh just the thought of Ninja vs. Dinosaur
Play the demo, if you enjoy the demo you'll like the full game.