Do they work...well...yeah, they work, but they may not give you the results you want.
The leg stretching machines stretches the inner adductors and inner hamstrings but doesn't stretch all of them. Most fighters need more than inner hams/adds flexibility to be able to fight well. The body also isn't using the muscles it would had the person try to stretch under their own power. This could lead to a sort of imbalance where the body is flexible enough to stretch far on one angle, but stretching far on a slightly different angle causes pain or injury. Keep in mind the angle that is used when your legs are splayed by the machine. Most MAists don't kick in such a way that keeps both legs in the same plane, yet that is the only angle that is stretched by the machine.
If it were me? I'd recommend saving some money...instead of a US$200.00 machine, invest in an hour with a good Kru or a personal trainer . Devote the entire time exclusively to learning how to - and how not to - correctly stretch and gain flexibility at ALL angles, and not just certain angles.