For what it's worth, I find the cheap four-sided ones (which are sold under a variety of names; I've seen virtually identical sets for sale from Faithful, Silverline, Lidl and Aldi) perfectly fine, although I don't do so much woodwork as some of the other guys on here so mine is probably the least likely to wear!
I use it for initial sharpening, then move on to some of those neat lapping films on a bit of glass to finish it off. In total cost me something like £30, and I have no problem getting a mirror finish (and more to the point, the chisels also cut really well!) I'm sure the DMT stones are awesome and all, but personally I don't think I have a need for them - at least not yet. And as such, my opinion on whether the cheap ones are a waste of money is that it probably depends on how much you're expecting to use them!