I use AXO knee pads. They provide very good protection and fit nicely into MX boots, however the foam/rubber padding tends to make you sweat a lot. I have only used them on long trips when the weather is cold (they do help keep you warm too) but would imagine that pads that have the cushioning with fabric over it would be more comfortable. Most skateboarding knee pads are like this and look like they would be much more comfortable.
[This message has been edited by Rowey (edited 07 November 2001).]