They're a dreadful idea and dangerous too - don't bother with them. Guinea pigs do hate being restricted - people who have tried them always report their guinea pigs goes in to huge fits as they try to get out of them. They are also independent animals that can't be 'led' in any way, and hate being pulled on - they like to run freely around within an area.
But most importantly they're not safe. A guinea pig has a tubular body and not much of a neck, so it is near impossible to male a leash/ harness really secure on their body without damaging the guinea pig (who have very delicate skeletons). If it's not 100% secure then it's worthless. Just make them a nice pig pen or run and let them do their own thing - that's what they love best!