I've been using NIK for several years. It has some handy tools.
As others note, Silver FX is worth trying. I use it often. I rarely use presets other than to occasionally "test" an image to see in different ways, but nearly always return to default and build my own custom. As in other NIK tools, working with control points is a novel way of adjusting locally, from brightness/contrast, structure, to shadow adjustment, etc.
I also like Viveza for colour. Again, control points come in handy.
I often use the output sharpening and noise reduction: Dfine. Efex Pro can, with restraint, create some nice images.
Now that it's free, what's to stop a person trying it out?