I use the Mac Yosemite built in dictation extensively, and I find it does a really good job of understanding my speech, at least when I'm speaking in complete sentences.
I also use the dictation commands that you find under accessibility in system preferences, Like scroll down and scroll up. (It even does a really good job of telling the difference between when I say "scroll down" as a command and when I say it as part of a sentence.)
BUT I can't for the life of me get any custom commands to work. For example, I have tried to bind "Close this" to cmd-w, and "tooth paste" to "success" (as an experiment).
I can't get dictation to recognize these commands, ever. I seem to be following the steps shown on every tutorial. It just doesn't seem to work.