I’ve never put Dragon Naturally speaking up against Mountain Lion’s built-in dictation that theoretically uses the same Nuance speech recognition engine. Macworld did this week, however, and it came away with some stark differences. They found the Nuance client software adds some niceties and does a better job at learning your patterns.
If, however, you want to dictate regularly, for more than short, occasional texts, Dragon Dictate is the way to go. The program learns from what you say and from your corrections, improving its accuracy over time, and also allows you to edit text using your voice, to control applications and much more.