I highly recommend the idea Smrobs posted about non-stationary curbs. I probably will never go back to a solid bit made of one big piece. You can't do a one-rein stop, and turning direct-reining can become confusing for a horse with a one piece bit.
I agree with Smrobs, fixed cheeks equal trouble in my opinion, I like the extra flexibility of the non-fixed ones, plus I love the idea that should I have to do a ors I can with the flexible cheeks. I rarely use a curb bit (most of the horses I get, I get them to train and by the time they are comfortably in a curb and going good, they are ready for a new home, and my personal horse prefers a snaffle picky bugger :)) but when I do you use a luw port with swivel cheeks.
^^ Ditto to what Spastic said. I usually put mine in the curb before they are completely consistent with neck reining but they are consistent and controlled at all gaits, have a solid stop, and a supple neck.