The legality is not still in question. It is currently Status: Approved. And I believe it will remain so. In fact, each one is shipped with a copy of the ATF approval letter.
Unlike the other "full auto simulating" devices such as the gat and hellfire trigger systems, the slide fire stock in no way modifies or even affects how the trigger operates. It doesn't have a mechanism to press the trigger for you. It doesn't change the operation of the trigger or any other parts in the fire control group. (trigger, hammer, sear, disconnector) all unaffected by the slide fire stock. You still need to press the trigger one time for each shot and need to do so with your own finger. That is semi auto any way you look at it. The ATF is not thrilled to do so I'm sure, but they have no choice but to allow it. If they ban it, then by extension they are also banning all semi autos across the board. Any semi auto can be bump fired. Even without the stock. Some are just more difficult than others to do it with.
Yes, I have one. Yes, it works. No, I will not sell it.