If they are allowed, has any one used them before? If so, what kind of success did they have?
One the the most inventive solutions I ever saw in FLL used pneumatics. I don't know if you where around in 2002, but in the challenge that year there was a bridge on one side that was about one third as long as the table It started up, but with by hitting a lever it would fall flat. Hidden behind that bridge was a windmill, and with the other things on the table, the easiest way to get to the windmill to push a lever, was to go over the bridge. The bridge was quite treacherous to navigate over, and a plunge off the side meant almost certain death for many FLL robots (lesson learned: ALWAYS PIN YOUR ROBOT TOGETHER!) So, one team, used the pnemautics to power a little cart that they sent over the bridge. It was genious! It required no additional weight on the robot, and if it fell off, you wouldn't accomplish that mission, but your robot was off doing other things, so why would you care? Now, the same thing can be accomplished mechanically. See
this video for further information (it's in the part with the gate). Overall, using carts to accomplish tasks is very effecient way to handle things.