As you know I have an X52, not the X55, but I assume the it applies equally. I own saitek rudder pedals, in fact I own the whole saitek yoke, throttles, gauges, switches, blah blah for when I'm flying a real flight simulator. I found the same as Rowander... precision in the yaw axis is not critical in flight sims, but it's the difference between a hit and a miss in ED, so even though my pedals are under my desk and plugged in right now, I still use the twist stick for yaw.
I used to own an old inexpensive Thrustmaster HOTAS that had a choice between the twisty stick and a rocker on the back of the throttle... back then I used the rocker and probably still would now, but I just wouldn't get enough accuracy out of the pedals.