Recently did a case study on this. They are moderately good at producing benefits in respiratory and metabolic areas, but do not lead to total VO2 output. Rather, they can help muscles of respiration and total body lactate clearing, increases in total workload, better effects of EPOC, and a few other benefits like decreasing inspiratory muscle fatigue.
Though it should be noted, the main findings of my case study were based of using a training mask that can induce hypoxia with a FiO2 content of 13-16%, and were tested in sprints 10x6s with 30s rest intervals for 4 weeks.
It's probably half decent just before training camp to get a lead kick start, and shouldn't replace aerobic work. I wouldn't be doing metabolic conditioning circuits, pad work, sparring/rolling in it, or road work with it. As the benefits of it may be modality specific (ignoring the idiocy of trying to fight in a hypoxic state).