3620 has one. It has been very effective in competition so far, and the Judges have liked it when our students demo for them in the pit. It uses a BAG/VP timing belt drive and slides on a linear bearing. Will look for some better pictures. Thumbnail below is from the week before Bag Day.
Team 2169 also has a gear slider, check it out at https://www.youtube.com/watch?v=cRelUdGfBqo. We use a vision program to align the gear with the peg in autonomous and it has proven to be effective. At our first competition we were able to score 8 gears in one match because of our gear slider and floor intake.
Team 610 has one and in all honesty its something that we have only really found useful for autonomous. It is not really a “slider” per say, as it has 2 pistons on each side that can centre the mechanism to the robot or shift one way or the other. We also have elastics to help centre the device naturally. All in all our driver hates having it move when he tries to line it up, and he always keeps it centred. We are at a point where we have now limited the range of motion by enough that we may just take the pistons off. Having a sensor now to detect when the peg is where it should be, illuminating an LED seems to be sufficient as a replacement for the pistons.