Like Wandering Jim, I first used O2 on Mt Fuji. It helps, no doubt about that. I've since used it on Shasta and Whitney and it was definitely beneficial each time. It's barely noticeable at low elevation, but its obviously helpful for the typical weekend warrior who hasn't had the days or week necessary to truly acclimate. O2 will provide temporary relief from mild AMS symptoms and give you a noticeable boost. It may only last for a minute or minutes, but doing this repeatedly over the course of a climb actually went a long way to make my hikes more enjoyable. There's no way to know if it prevented more serious symptoms, but it certainly helped my energy level and enjoyment. It's a little pricey, but it does help in a positive beneficial way, without any side effects like drugs can have. Just don't expect miracles or think this is a cure for serious altitude problems. As Harvey wrote, you shouldn't consider it as a treatment for serious HAPE or HACE, but that's just a matter of managing your expectations. If you're that sick, turn around fast, period.
