I saw this on Amazon and thought why not. We have a Dyson (DC14 Animal), and so an extra attachment to add functionality sounded good. Basically it's similar to an attachment we already had, but is about 3 times wider. So it sucks up more in less time. When changing the sheets I thought I'd try it out on our matress and protective cover. They appeared cleaner after passing over (but it's hard to tell with grey/beige), and you could see more dust in the Dyson, so it must have done something!
I wouldn't say it's a must have, as you could do the same with the standard accessory (taking much longer). It's good to see you can get these bits at reasonable prices - and since they fit all Dysons you could lend it to friends/family to use as well.