This article is from the Impotence FAQ, by Sine Nomine llama@drizzle.com with numerous contributions by others.
They seem to work for most people. Common complaints are lack of
spontaneity and the hassle of messing with the equipment. A few men
find that pumps don't produce an erection that's as hard as they'd
like. Also, applying vacuum too quickly can result in pinpoint
hemorrhages or bruising. Some complain that the constriction rings
inhibit ejaculation.
 
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