I do a lot of black and white and felt it was very nicely designed, has a good and pretty convincing set of features, but is significantly overpriced.
If you know how to do b&w in Photoshop CS3, I don't think Silver Efex Pro adds much at all unless you routinely add exactly the same filters AND clarity / wide area sharpening AND grain AND can't figure out actions, layers etc. I don't think the end results are better in an absolute sense - it's just nice having a bunch of b&w-oriented controls bundled together.
If you don't know how to do these things in Photoshop, then it's a nice shrink wrapped solution that will have you going round proclaiming "you can't do that in Photoshop, you need a dedicated solution". Hm.
I felt the grain was particularly attractive, though with these b&w apps I always wonder about how accurate the film recipes actually are - is that HP5 in ID11 or Perceptol or how about Rodinal? Maybe these recipes are spot on, and maybe there is some creative sense to mimicking film....
Overall, I just didn't think it was good value for money. But the earlier poster was right - "All one can do is demo it for yourself and then decide".
John