I have been using these HF diamond stones for a few years and had acceptable results. I can really only use them for the side wall (or perhaps the occasional free-hand work) since they won't fit in my bevel guide for the bottoms.
Let's face it, nothing is free. There is a large quality difference between these from harbor freight and the other ones I have from SlideWright. The cheap ones wear out really quickly maybe due to a cheaper process in attaching the diamonds. But, they are really big so even if one area goes dull, you can move to another part of the stone. I actually tried to cut one of these down so that it would fit in my base bevel guide and it basically self destructed. I concluded that it might not be possible to cut these successfully because the metal is sort of molded to the plastic. Cutting it invariably will break the hold / attachment and make it pretty much un-useable.
I am just more careful with my good Diaface ones . I always use them wet (when I am working I have an old margarine container with water in it to dip them in) so that the diamonds stay where they should. The good ones also seem to cut better if that makes sense. They might feel the same courseness to each other, but the expensive ones are just more effective for some reason. That being said, you have to stay on budget and these things work, but the lack of ability to use on the base hurts.
As far as the stones are concerned, it really depends on what you want to use them for. It is always nice to have a pocket stone available for removing rust or taking care of a hardened damage spot when filing. I think these would do well for that. I have never used stones in my bevel guide for polishing / hardening edges though. How do the stones work for you?
PS - I use Harbor Freight for lots of ski tools and supplies when I can. Here is my Base Welder which does the EXACT same thing as the $350 fancy pants ones you see in the shops.