In my experience, they don't work well. The pads always seem to ride up over the knees every time it's bent. The other side of the coin is if the pant leg is too long then they're not covered when the leg is straight.

If you want cool, practical field pants, you can get lightweight BDU pants. The ones with ripstop fabric are the most breathable and they're surprisingly durable(plus cargo pockets). For something more athletic and even more breathable, I really like athletic pants that are geared more toward rock climbers and hikers. Comfortable, belt loops, and plenty of zipper pockets. Those are my #1 choice for riding.