For the PSX64, response is pretty spot on - this was something I worked on for a while. When you plug in a controller, it automatically does a timing auto-calibration to see how fast it can poll the controller before it starts getting crap back (each controller can only be clocked so fast) - so you're always guaranteed to get the fastest response possible for that controller.
Also, I know one person who bought two and hooked them both up to his Minimig - he had a pretty cool setup, he posted the pictures around here somewhere, I will look for them.
And it works fine with the X-Arcade, I tested it out with the X-Arcade TankStick