I climbed quite a bit on the fly and tried several different cords. They all worked the same as on any other rope, nothing different really. Use whatever you always liked on other climbing ropes.
That's my two cents on that.
Jarod, the problem with cords with eyes spliced on them is they are hard to adjust the length. You can add a loop or cross, or take away a loop and add a cross, or whatever, but the same length cord and hitch combination on a 1/2" rope, will not work the same as on a smaller rope like the Fly. If you were able to adjust the cord length just sligthly shorter, it would grab better.
That's my two cents on that.
And since everyones always gotta throw their opinions in on a thread like this, my favorite cord is 5/16" Ultratech.
That puts the total value of my post at 6 cents. Sorry, no refunds.