What Outerspace said, basically, they are a good distraction. I was lucky enough to be given one when my friend no longer needed it, so I will use it again for this one.
I bought pads on ebay, I only had one set. They are super-sticky, so I reckon that even if you took it off to have a bath or something, you could put it on again afterwards with the same pads. Hard to know whether you'd need extras, as you don't know how long you'll be in labour. (Fingers crossed, it'll be all over in about 20 minutes, right?)
I'd check the prices of buying vs hiring. You'll never use it for anything else (except maybe getting your own back on DH), so it depends whether you mind storing it. Of course if you do buy one, it doesn't matter if you buy extra pads and never use them, because you'll have them for next time.