Assembly language is an art. It takes time and effort to master it. Depending on your goals, your availability and dedication, it might or might not pay off on the long term. Because the main interest of assembly programming is to build the guts of operating systems, drivers, interpreters, compilers, and games, where C compiler fails to provide the needed expressiveness.

You may need to use assembly, or you may want to use assembly. In short, main practical (need) reasons of diving into the assembly realm are small code and libc independence. Impractical (want), and the most often reason is being just an old crazy hacker, who has twenty years old habit of doing everything in assembly language.

You may want to check Assembly -HOWTO or Paul Hsieh's pages for more educated opinions.

My personal opinion is that Assembly enriches programmer's experience, and gives a new meaning to the world of computing. I got this Assembly frenzy several years ago, and I can spot some reminiscences of the old days even today.

My main Assembly project - besides two bazillion other micro-projects - is D3X, a 32it DOS eXtender.