I've used the Nik plug-ins since version 1, about nine years. Generally, I've had very high praise for the software. I currently own the current versions of all the plug-ins, but I have a big caveat. As they've become more sophisticated, they've become buggier, especially in PS CS4. Today as I've been working, Silver Efex, for instance, has crashed no less than five times. Each time I've had to restart Photoshop -- either to get the plug-in working again, or because it's locked up Photoshop (as it just did two minutes ago). Once I've had to completely reboot my computer. (I run Windows Vista32, so I can't speak for Macs or others). I just know the plug0ins were more stable under CS3 -- though I had my share of problems with Dfine.
Those complaints aside, I've found the sharpening to be among the best, and Dfine the noise cancelling, to be excellent. As for Silver Efex, it's a good program, but not significantly better than, say, Alien Skin Exposure 2 (which I also own). Just a different tool, but one that, at the moment, I want to throw out the window!
Nemo