You cannot fire it folded is the only downside. But for storage its great. But who really cares about how small their rifle is when they arent using it?
I would be interested in hearing from someone with experience. Lengthening the receiver extension and then adding mass to the BCG to account for that extra length would lead me to think timing and function could be effected. Not to mention the bolt extension being held on with a spreading tension action does not inspire warm fuzzy feelings. A piece of potentially loose metal rattling around in your upper would ruin your day quickly.