A few people have asked me about my opinion on altitude masks. Unfortunately all these devices are designed to do is restrict the amount of air that you breathe in with each breath. The physiological adaptions that can be achieved from spending time at altitude (usually 1200-1500m above sea level) are not achieved because of a reduction in the inspired air but because the percentage of oxygen in the inspired air decreases.
Altitude masks do not alter the percentage of O2 you inspire, only the total volume of air. So whilst training with altitude masks may benefit you in terms of your mental toughness because training feels harder, you should not expect much more. In fact, you may wish to consider that altitude masks will actually prevent you from training as hard as you could without them and may therefore be limiting your potential for adaption.
On the other hand there is a device available known as powerbreathe which has been shown to improve the conditioning of the respiratory muscles and endurance performance. As it is used independently of training you can use it without limiting your working capacity in the gym.