I have used them and have also made/used a home-made version. I manage a training studio and I have access to equipment when we are closed, so I channeled my inner Vanderlei Silva!
I grabbed a snorkel and stuck a wine cork into the end to block it off. I then used a screwdriver and drywall screw to make 2 holes in the upper part of the snorkel. This saves you $100+ and gives you the same benefit.
Interval cardio with that snorkel in is TOUGH! It is hard to keep your body from sucking in air through your nose, but its worth it. I try to hit 1-2 cardio sessions per week in this manner for sports-performance.
Using the snorkel, at least if you don't like it or don't stick to it you don't have an expensive peice of workout gear collecting dust. If you like it, you can buy a real version. Bas Rutten has the cheapest version out there at $50
http://www.amazon.com/Official-Bas-Rutten-O2-Trainer/dp/B004V6R9XA . The mask version seems like it would stay on easier though....