I don't know about safety, but I would NOT do it!! They do NOT work on making your penis bigger. I read that they really only work for old men who need an erection but that seems kinda weird to me
'Course I don't specialize in old, wrinkly penises
Basically Wikipedia says that electric penis pumps can cause damage to the penis, over-stretch it, and burst blood vessles. Hand pumps are usually used for masturbation (although it'd be better to use your hand anyway...), and it also says that if you are to use one, you should READ THE INSTRUCTIONS AND FOLLOW THEM, risk losing or hurting your dick. If you pump too hardly with a regular hand pump, it could damage your dick perminently.
Although, it is used to get an erection and this confirms that old people might use it for their own problems...
...and whatever you do, don't use a vacuum cleaner on it! Weirdos 0_o jk