I knew someone who used one for preparing for high altitude racing. We both did the same race. I did not use the altitude tenty thing, as it seemed too much a pain. Instead I went to the race 7 days prior to try to acclimate and used my heart rate monitor to gauge myself. My friend did well and took 5th. I ended up with one of my best performances taking home stars and stripes.
Interesting enough, the same week, another friend went out to leadville 100 and tried the show up and race and trick your body tactic. He ended up in the hospital.
So in my impression, these altitude systems can certainly help out with high altitude adjustments, but knowing your own body and eating and resting carefully play just as important role.
As for just using them for a boost, I do notice some performance difference for about a week after being at high altitude. I may not feel stronger climbing, but I recover a tad faster from a hard sprint. However, using them on a reguar basis, I have no experience with. I always hear "sleep high, train low". Yeh, I know anrothar endorses sleeping high
Too much high altitude, and you lose power. Plus I think your wives and girlfriends may start to think you are crazy sleeping in a tent every night.
