The most basic control device Descent offers is the keyboard. You can combine it with a mouse if desired. Most highly skilled Descentrs however play with a joystick (or even two ;).
This is the stick of choice for all right-handed people. Although it has a twist handle giving you an additional axis of control, you will still have to combine it with keyboard controls.
If you are left-handed, imo this is the joystick to go for. It is very sturdy, and you can adjust to right and left-handers and to a great range of palm sizes. It also has a twist handle plus a throttle control. Above that, it comes with 10 buttons (3 on the handle, 6 on the carcass), two of which function as kind of "shift" keys giving you a total of 16 button controls.
An unusual, but perfectly 3D-suited controller is the SpaceOrb (if you manage to get hold of one that is). This is a controller giving you full 3D control. Its basically a crescent-shaped gamepad with a ball attached to its concave side. The pad itself is bearing four control buttons aligned so that you can easily reach them with the fingers and thumb of the hand you are holding the pad with. The other hand holds the ball. The ball can be moved and twisted in all three spatial axis'. Its a bit hard to handle though, esp. in the beginning, and you really need to get used to it. I once owned one, and I found it not having a firm support but having to hold it in one hand being a major backdraw for fine-controlling the Pyro. There are "Orbers" however who have brought it to quite high steering skills.
What I have heard from some real good pilots is that the ultimate Descent controls were a complete flight control set (joystick, throttle/missile control, steering pedals).
This is a pretty new and very promising 3D controller built for 3D drawing and editing that looks like it is perfect for Descent. On Mac OS X, you can use ControllerMate to interface it to D2X-XL without further ado. On Windows and Linux I would need to integrate the controller via a publicly accessible SDK, which I however haven't yet come around investigating into.