So speaketh Dr. Drew:
Q: I keep hearing an ad on the radio for penis pumps. Do they actually work? I suspect not, or else every guy would have a 9-inch penis. But if they do work, what are the drawbacks and side effects?
A: You've got that rightnot only would every guy have a 9-inch penis, but these devices would probably cost as much as a car. Men who have difficulty sustaining an erection sometimes use a device similar to a penis pump, and there's also a device that clamps a ring around the penis and prevents blood from escaping. Penis pumps cannot increase the size of your penis regardless of what any ad tells you. I don't know of any long-term side effects from using a penis pump.
http://drdrew.com/faq.asp?id=1524
(me believes dr. drew - you on your own)