I can't (and won't) speak for the men on this, but I have used a pussy pump frequently in the past. The effects for me were temporary, lasting no more than twelve hours. There was no permanent change to my labia in any way.
I have read warnings about cock pumps in various books and magazines. The authors recommended using them for infrequent masturbation sessions lasting no more than twenty minutes. If used incorrectly or for long periods of time, the blood vessels become permanently damaged possibly preventing future erections.
My hubby has used them and then do stretch the skin on the penis and cause it to extend while using the pump. Some men may find this pleasurable... however the ol' man said it was just weird. Also, as soon as the suction of the pump was stopped, he went instantly back down to normal size. They work just like vaginal and breast pumps work. They stretch your skin making things appear bigger and might be used as a device for play... however... I don't think any of these products have the ability to make your cat, boobs, or shaft grow unfortunately.
They DO NOT work, as someone who used to work in a sex shop I've had to explain this to a good number of disillusioned men (and their wives/girlfriends) who've come looking for a non-surgical enhancement.
The way a cock pump works (or any genital pump for that matter) is to apply suction to the organ, drawing the blood into that particular area. Then, with the pump still on you and pressurized you slide on a cock ring to trap the blood which in theory makes for a more solid erection. However because your penis will never be able to expand beyond it's natural size/volume and because cock rings only limit the flow of blood, they don't actually stop circulation to the area, your body will "equalize" itself within a few minutes.
The same erection could be achieved by taking Viagra or another pharmaceutical designed for this purpose.
Do they make a difference in size compared to a flacid penis, yes. As far as comparing the pump induced size to a ED drug assisted erection, in my case definatly not.