I had to really hunt on the Rogers site to find anything, but I eventually did find the right page. It sounds decent but I do note that emails are limited to 20 words. The rest of the limitations sound reasonable though.
I would guess that like other voice-command type apps, that it's done via servers (otherwise, why would be be charged a monthly fee?), and if we had unlimited data plans, I'd consider it but for now, I will pass. In my case, I don't drive, so it'd have less value for me personally, but I can see the benefit for having such an application for while driving and such.
I'm on Cingular, not Rogers but I do use the Nuance Voice Control application and it's outstanding. I drive a lot for my job and it's great to be able to send a quick email or a quick SMS from my phone without looking.
Another great thing is the feature for adding a calendar entry without typing. One limitation is the 20 word email thing but it doesn't bother me too much as I'm not using this to write a novel. One great thign would be a single button verification (say the same one used to start the voice control) and a single button cancel button.