Revitive machine

You should not use one if you have had a joint replacement. It will loosen the cement over time.

I would seek medical advice and if permitted to use one I would try to find one that is not being used for any reason. It would save you money.

My husband has high blood pressure so cannot use one of these so I bought him some compression therapy boots for the bottom half of his legs which help with circulation. They vary in price and ours were under £50 but are very good.

I would be cautious to be honest. There are some medical conditions for which hands-on massage are contraindicated and the same could apply to these machines.
If you're intent on buying one it would be wise to speak to your GP first.
For what it's worth I think they are a waste of money and simply a lazy alternative to proper exercise and movement - my physio agrees too.

I think they can be totally unsuitable for certain conditions. You say your DH has ‘sore legs’ and you have a ‘sore knee’, but that’s very vague. Perhaps you should ask at your Surgery if a Revitive would help.
My BinL swore by his for neuropathy and spent hours resting his feet on it. My DH felt no benefit for the same condition with his. I bought DD2 one a couple of years ago, but I don’t think she’s used it for ages now.