Don,
I currently use Topaz Adjust, Topaz Detail, Topaz DeNoise and Topaz Simplify, Nik Viveza and Silver Efex Pro. Each plug-in has its own uses and specialities. For me, I especially like Topaz Adjust to bring out contrast and enhance detail and colour, and I use that when I want to make an image really pop. Much of what people assume is HDR processing in my images is done strictly with Topaz Adjust (I never shoot HDR).
Topaz Detail is newer to me and I find its the best sharpening plug-in I've used to date. Part of the appeal is that it sharpens (naturally!), but also allows for colour and contrast enhancement as well. Many of the images I would have used Topaz Adjust on before now get processed in Detail. This is because although Adjust does sharpening, it can lead to halos, whereas Detail can give me very similar colour and contrast effects as Adjust, but sharpens without the halos.
Obviously with NEX files, I don't use Topaz DeNoise much at all, but it does very well for my higher ISO m43 images. Topaz Simplify is more fun than serious, but sometimes it will yield some interesting and very usable results.
Many of my B&W images have been processed in Topaz Detail first, to sharpen obviously, but also to increase the contrast and in some cases bring out the colour, so that when I then go into Silver EFex Pro (SEP), I've got a larger tonal palette to then convert to B&W. I'll then typically use the control points in SEP to selectively burn/dodge for the final image.
One thing I like with the Topaz products (and SEP) is the presets, or rather that one can create your own presets. This makes it very simple to apply different "looks" to an image to quickly judge what might look best.
Hope this helps, but if you have specific questions, ask away.
Cheers,