All the time. Wouldn't be without it. Now that Google has bought Nik and combined the plugins into a package it's even easier to install and uninstall, which is important to me since I have to move my desktop license from Colorado to Florida and back again each year. Silver Efex Pro alone is worth the price of admission, and so is HDR Efex Pro. If you ever need to make local color adjustments there's nothing else out there that can beat Viveza. And Sharpener Pro can save you hours of tedious work.
There are a bunch of tutorials out there on the Nik collection. See http://www.video2brain.com/en/lessons/nik-sharpener-pro
, for instance, which gives you a ten minute rundown on Sharpener Pro.