Admittedly I've not played much with it but on my initial testing I found a lot of the effects are quite gimmicky and if you have a basic understanding of adjustments, layers & actions in Photoshop you can do most of them yourself (and do them better with finer control & subtlety).
Use it quite a lot. It has some good effects on it but I go against the general opinion and don't find Silver Efex that useful or better than anything else. If anything I prefer Imagenomics Real Grain and Perfect B&W.
A preferred suite for me is Perfect Photosuite 7, this includes Perfect mask, Perfect Portrait, Perfect Effects, Perfect Focus, Perfect B&W (excellent) and a resize programme. Of course many of the actions can be done in layers/masks in PS but Perfect Effects has some striking styles and great borders and Perfect Portrait handles skin/blemish enhancements really well.....(just waiting for the backlash on this!!)
My mistake was not buying a 64 bit machine as these are memory heavy particularly when used as a PS plug in!
I use Nik and love it, though mostly for my wildlife shoots. Whilst I wouldn't necessarily say it is better than other products I think it wins out in cost/benefit ratio (every time I get a cut price offer on Perfect Photosuite I hover over the buy button but don't quite ever click)
Highlights: Silver Effects Pro is excellent on the rare occasion I do BW. It has helped me out no end this winter with the drab flat days with tonal and contrast adjustments to give that little extra lift. I find things like soft focus and glamour glow adds to portraits/model shots as long as you rain them in a bit. I wouldn't now be without the noise reduction and output sharpener.
Like all effects suites I think you might end up using 4 or 5 regularly and some of the wackier ones occasionally.
As already stated I wouldn't get into a fight over Nik verses PP, either are worth a punt.
Silver Efex has an excellent menu. It's quick and easy to use and equal to any thing Photoshop has to offer. for the small investment it's always useful to have. As for the others in the suite Viveza is a real star if used well..