Well, not quite "all powerful". You can't affect anything in the real world (barring a few weird occurrances that would best belong on the "Beyond" board)... But you can do anything, be anything in your dream world--like being the author of a story, or the director of a movie.

EDIT: Actually, now that I think of it, you can affect things in the "real world" by affecting yourself--for example, lucid-dreaming about speaking to an audience to help overcome stage fright... overcome nightmares by taking control of them... and of course you can wake yourself up.