Training & Nutrition - Your advice on powerbreathe thingies sought

Has anyone used one of those powerbreathe type devices that restricts the airflow into your lungs so that your lungs have to work harder?
The idea being that your lungs get stronger so find it easier to work when you're exhausted, improving your aerobic capacity.

If so, what was your impression of it? The reviews on Amazon seem pretty positive.

Read:

http://home.hia.no/~stephens/exphys.htm

...specifically, this for an answer about lung capacity:

http://home.hia.no/~stephens/ventphys.htm



Ventilation Issues in Endurance Performance

Is ventilation volume a limiting factor to maximal endurance?

Sometimes you hear people say "I ran out of wind." Is that really possible? Can we reach a point in exercise when ventilation just can't keep up with demand? The answer is no, assuming you don't have acute asthma or some other severe pulmonary dysfunction. We can measure a person's maximal voluntary ventilation (MVV), the maximal volume of air they can breath in and out while at rest, and compare it with their maximal ventilation during exercise. What we see is that untrained people only use about 60 to 85% of their maximum ventilatory capacity even at maximal exercise. For example the MVV for an average male might be nearly 200 l/min. However, during a treadmill VO2 max test, they reach a peak ventilation of only 140 l/min. Highly trained athletes use more of their capacity, perhaps over 90%, but ventilation capacity is still not a limitation on performance. Unlike the story with cardiac output, even during maximal exercise, the ventilatory capacity is not maxed out.

Link please?

Couldn't you just breathe through your nose?

Some people have mucus problems in the nose, making it stuffy while riding.