I've never actually played a Drakengard game, but after Nier I'm intrigued. Should I go back and play the first two? Is the gameplay as bad as they say?
It depends on the "they" you're thinking of.
I'd call it a mixed bag (overall solid, though) of oddly placed mechanics and fun but (pretty easily) repetitive genocide.
The magic is really neat to experiment with, and dragon riding is hit or miss.
D2 fleshes out its story and present better than D1, and makes some gameplay improvements, especially that dumb camera from the first.