2-Shot Trigger Groups For AR?

I've been considering purchasing a drop in "2-shot" trigger system for my AR15, but have heard mixed reviews. Does anyone have experience with such systems and their utility? Naturally use would be limited to plinking deep inna'woods or at a range savvy to such technology. Also, my understanding is that there is currently no tax stamp requirement for this item, has anyone caught wind of impeding changes derived from the current gun grab initiative of the month?

E.G.

*Echo Trigger

*Binary Trigger

*Digital Trigger