OK - my own fault for opening that one up.
There is a reason athletes don't train with them, but at altitude instead. Airflow restrictors just suffocate you rather than imparting the benefits of altitude camps (more oxygen-carrying componentry in the blood).
I would hazard a guess that most people have no real problems with the strength of their intercostal muscles or their diaphragms, but with their lungs' ability to transfer oxygen to their blood, etc. If you're a former smoker, the issue is with the crud in your lungs rather than the strength of your muscles working them.
Based on your other thread about core exercise via machines, I am going to tell you to get one of these airflow restrictors so you can enjoy purchase justification syndrome: you will have spent money on something, giving it inherent value to you. However, it won't do anything for your fitness and will probably put you off cardio workouts because you will feel like death (because that is kind of how these things work).
Better to just save your money and go swimming or cycling (or just even walking), but enjoy, regardless.
