This switch kit is a very good tool to have for your X series products from Surefire. It will work on the older X200, the new X300, or the X400, which has the laser on it as well. It has a master switch on it, to shut the light off and prevent unwanted activation. The rail mounted controller has a push button on/off switch, and a pressure pad, and the controller mounts easily on a rail system.
One thing to be aware of is the length of the cable - it's quite long, so if you have a shorter handguard like a 7 or 9 inch one, you'll have to have the controller on the other side of the handguard, as the cable is too long for same side mounting. This isn't a big issue, as I find the optimal set up for a right handed person is the light on the left, and the controller on the right, for activation with your left, support, hand.
The controller unit is removable from the light switch, and you can install a pressure pad only switch, in case you want to have the pad mounted on a foregrip.