I was excited for this feature, but I wish it worked locally on the machine rather than through Apple's servers. To be honest, I would trust Google more with recordings of my voice than Apple just because Google has been vetted by the public and media for privacy concerns for so long, and Google's privacy policy is more explicit than Apple's when it comes to what each company does with data. Apple is very vague on what it can do with these recordings.
Plus, I've had experiences with MobileMe in the past where I've had e-mail issues, and I'll be chatting with their outsourced support and they've asked me if I see a certain e-mail in my inbox, letting me know that person is seeing the contents of my inbox. It was kind of surprising, at least at the time.
I'll probably use dictation, but not for personal journaling, which I use my Mac for pretty extensively.