I havent used one (duh! ) but to my understanding technically they will bring the blood flow directly into your penis with each pump therefore you will have a full on and as hard as possible erection, however maintaining it will be completely on you body and mind.... in theory it should work but theory and practice dont always go hand and hand.....
I dont believe in cremes, pumps, pills, weights, jelkin or contraptions on this one, I think surgery is the only real well known proven alternative....