Great tips for using dictation under Mountain Lion or a Siri enabled iOS device from Macworld:

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Enabling Dictation

  • Visit System Preferences and look for the Dictation & Speech in the fourth row of settings.
Enabling Dictation in Mountain Lion Step 01
Enabling Dictation in Mountain Lion Step 02

Once you've turned it on, you can decide what shortcut key to use to enable Dictation.

Sample dictation

As you can see from my tests below, it's not great. It even conveniently "missed" the isn't in the first sample.

The dictation is great but it would work in a pinch.

 I use dictation to write this part of the post

 As you can see it forgot the D and Mr. Cents and I can say. Again.

 Attempting to say. This while you're dictating doesn't work very well because it will put it. Very sad.

Mountain Lion Dictation Tricks