As with all plugins, it has pros and cons. But, generally I like the plugin and use it just to ckeck things out after making a skin tone change. If I warm up a particular subject Skin Tune will make the slight changes that, while keeping the warmth, make the skin look correct. It likewise helps with radical skin tone changes (like adding a heavy tan) by cleaning things up and making the skin tone correct.
I've also used the plugin to make overall color cast corrections (this surprised me at first but is obvious now). Since we have a good feel for what skin tones should be in an image, correcting the skin tone with Skin Tune will often bring the rest of the image into line if there is a general color cast problem. Anyway, it usually gets me close and then I work from there.
The cons: it is an 8-bit plugin, it bruises the image so you have to work from a Merge Visible layer if you've already done some work, and the interface is not too intuitive (e.g., using the scroll bars after selecting a skin tone and race (e.g., Asian) will change the skin tone up or down one value rather than just show you what is available.
Try it out and let me know what you think. The demo is the real plugin that will modify 20 images (BTW, if you work on an image and then cancel that doesn't count toward the 20).
I also have their MD2020 plugin and don't find it very useful at all, but maybe that's because I'm missing something. It just seems to be too coarse and hard to fine tune.
Bottom line: I like Skin Tune and recommend it if you do a lot with skin tones.
Hope this is useful.