I don't think it will be THAT bad!

It shouldn't effect battery life - it isn't a service that is constantly running in the background. The only time it activates is when the user activates it with the Home button.

Per my suspicions, it appears to not be a perfected technology yet. I watched the keynote last night and Schiller made a point to warn the crowd that it is a beta app yet - but the demo went off without a hitch. They also made a point that it is being released in beta form - that is an "out" for them if it may not live up to expectations. Sounds like the service "learns" over time though - for people's different speech patterns. Regardless, if people find out it just doesn't work to their expectations, they can simply choose not to use it - it's nothing that is "in your face" all the time.