Izzy has one of the older versions of the Premier Equine magnetic rugs. He wears it while travelling mainly.
I don't know if it makes a difference really, except that he travels really nicely in it and arrives calm and happy. He gets off the lorry ready to work and never needs long to warm up. But whether that is due to the rug or just him - who knows. My osteo lady says that it probably does help him and he is in good condition - no sore spots, loose, supple, great lift through back etc - so she says to keep using it. Think it is good for after endurance rides.
My friend has an ex-racer who has an SI injury and is very tense behind and on his back and she has borrowed if for him and said she notices that he is a lot looser and feels more supple and ready to work when she gets on. She is going to buy one as she definitely think his back is better for wearing it.