I have the Nik Software Complete Collection for Lightroom, and I've found that I use Color Efex Pro a lot more than Silver Efex Pro. Silver Efex Pro provides some nice templates for B/W conversions, but I've found that the B/W conversion in Lightroom coupled with the channel mixer to provide me with enough control over the B/W conversion process. As such, Silver Efex Pro is rarely used.

Color Efex Pro on the other hand, has loads of useful filters that produce effects that cannot be done using Lightroom alone. I do lots of portraits, and I've found it an invaluable tool.

This is an example that was processed using Color Efex. The skin was softened, eyes were enhanced and some touch ups were done to the exposure. While eye enhancements and skin softening can be done in Lightroom/Photoshop, I've found that Color Efex produces the most pleasing effects (i.e. not too over done) with minimal effort.

It's not just useful for portraits. This is another photo that I edited in Color Efex to bring out the autumn colours while leaving the subject (the lady pushing the pram) untouched.

In summary, I cannot recommend Color Efex enough. It's a fantastic bit of software for your toolbox. I know that this is not part of the question, but if you're after another plug-in from Nik I would highly recommend Viveza.