It will depend on the price really, and whether the camera is what you're after. That is to say, bear in mind that digital SLRs are a lot like computers, there is a certain amount of technical advancement with each new generation therefore I would expect to pay significantly less for a camera that isn't current technology. Then you also throw in standard depreciation for an item that is fairly complicated both electronically and mechanically, and you would expect to pay even less.
Unfortunately as has been debated here before, second hand prices in Singapore seem extremely high in general, but that doesn't mean you might not be able to find a good deal somewhere, and if so then yes, assuming you check the camera out as thoroughly as you can (bring a *properly* experienced friend along to check) then they're worth buying.
The key really is the price, and saving 20% on anything that is second hand and not in warranty (I think most manufacturers' warranties are not technically transferable) is the bare minimum you'd expect to get off, and more if the camera has any sort of age. If it's not current generation then I'd really expect a sizeable discount.