I've been using the original Belkin n52 Nostromo Speedpad for several years :
The colors aren't very sexy compared to the more recent models, but it works perfectly fine. I hesitated before buying it, fearing it would be another one of those gadgets that end up taking the dust on a shelf, but since I tried it I simply couldn't play without it! Unlike a keyboard, no matter how "gamer" they call some of them, this feels like an actual gaming device. It is so much more comfortable than a regular keyboard, and the directional arrows are very handy. I like mapping jumping and crouching movements to the up and down arrows, moving in 3D space this way feels very natural. I use the big black button as a modifier. Any button can either toggle between the four modes, or you can use it as a modifier while you hold it. You can pretty much customize it the way you want it, with each key mapped to up to four different functions.
Anyway, the keypad is a great addition to my HOTAS setup, like someone else mentioned here I use it to navigate the UI, both in flight and on stations. It will also prove very useful when we get to walk out of our ships! I used it like this on X Rebirth, it was perfect. It sits nicely next to the throttle, symmetrically to the way the mouse sits next to the stick, unlike my keyboard which has to rest awkwardly behind the HOTAS...
I definitely recommend getting a keypad to anyone who plays with mouse/keyboard on PC, it's a worthy investment. The Razer keypads are essentially updates of this old model, and look pretty good. The Logitech G13 on the other hand looks very weird, the shape and button placement seem very awkward, I bet it's no way near as comfortable as the original Belkin design.