Extensive? No ... less than a year ... but I like it a lot.
It is easy to use, easy to learn and produces excellent results. My very first attempts were surprisingly good.
That said, I don't think it does anything that can't be replicated in Photoshop.
I've used it for less than a year, as well. For almost all images that I would convert to B&W, I use the NIK program, which has immense flexibility with ease of execution. For some images I still use the "Black & White" adjustment layer in CS4, or in the RAW converter (essentially the same variety of settings).
That said, the NIK program offers several varieties of control (contrast, brightness, "structure" which seems to be a local contrast feature such as Clarity in PS, graininess, "photo filters", a check on the distribution of tones, film emulation, vignetting, etc, all available simultaneously without having to leave one set of adjustments and open another. In addition, the built-in options of brushing the effects in makes things simpler than setting up similar options in CS4.
Could I continue to function without Silver Efex Pro? Certainly. However, I know that many of the conversions I have done would not have got done in CS4 alone, even though theoretically they are all possible. There is another factor in using the NIK program, which really is set up for photographers: Going through the vertical "filmstrip" of presets on the left side of the screen never fails to stimulate ideas for treatment of my image, andy any combination of preset plus manual adjustments can be saved as a personal preset.
I find that Silver Efex Pro and Color Efex 3 allow me to put finishing touches on images that I used to be happy with. And while the Color Efex filters can produce startling effects for those that like them, they can also produce very subtle but powerful effects with ease, in an intuitive way not easily found in CS4.
No, I do not work for NIK.
