My wife and I messed around with one last night (first time I tried one)... INSTANT wood, and in my case it stayed that way the whole time. It did hurt because it was pulling/pinching the sack, but not a big problem. Bottom line it works to get a fast stiffie, but as others said, the pump will NOT increase actual size. The penile suspensory ligament governs the length, and IF it can stretch at all, it's maybe a quarter inch.
TIP: The vacuum pump they sell you on toy sites for as much as $200, is the same exact pump they have at Harbor Freight Tool as a "fuel pump & vacuum tester" for $20. Anybody who wants to get hooked up, might want to get the pump at Harbor Freight and the penile cylinders only from an adult site.
Charles