I've spent countless hours playing with the trial versions of these tools for over a decade, long before Google took over Nik. But the price was prohibitive.
Sharpener was always the most useful. The earliest version worked very well, but had too little control - not something you can complain about with modern versions.
Color Efex has some very nice filters. Brilliance/Warmth was always a favourite and White Neutraliser can be very useful.
Dfine promised so much when it was launched, but never delivered. I quickly discovered it was vastly inferior for Noiseware, which has an uncanny ability to preserve detail while obliterating as much (or as little) noise as you want.
Viveza is useful for some types of tweaking. But for me the rest fall into the novelty category.