I got one as a wedding present, think it was a Tefal one, but it was from John Lewis anyway. White plastic with three interchangable metal plates with different sizes of holes.
True, it makes lovely smooth mash, although I still found you needed a bit of butter and / or milk or it was a bit dry and difficult to spread out over things like shepherd's pie.
And yes, you save preparation time in that you don't have to peel the potatoes before boiling them because the peel just comes off in the ricer.
But, if you're cooking for more than about 2 people it is a king-sized faff to then take each potato in turn and put it through the ricer, remove the skin, take the next potato, etc. You either burn your fingers trying to do it quickly or accept that your mash will be stone cold by the time you've finished ricing it.
I gave mine away about 4 years ago and have never once wished I still had it. Personally, I'd save my money and just spend more on really good mashing potatoes. I don't put much butter in mine and just a bit of hot (skimmed) milk, but I think the key is to have good potatoes cooked for a long time and after you've mashed them and put the butter / milk in, beating them like mad with a wooden spoon.