One of the most important things about Google Now is it's all opt in. You have to enable latitude. You must enable location history. You have to enable web history. You have to enable/opt in to nearly everything. To me that was a huge point that everyone has over looked. Up until I started using google now in ics I opted out of all tracking from google (except location for maps and weather). Most people enable web history or maybe latitude. So it's not a big deal but I've also left it disabled simply because I didn't need it. It's great to see Google preemptively concerned with privacy (if only to help stay it's legal troubles).
On the Siri vs Google Now, in my opinion Google now isn't a true Siri competitor and I don't think that's their goal. Google now is an integration of (nearly) all google services offered. it's what they've been slowly pushing toward since the release of Google+. In many ways it and Siri can do similar things, natural language voice recognition. Set appointments, email people, do daily tasks. At the same time though Siri is suppose to be a personalized assistant. Google now give you access to all Google's services relevant to you in a quick and easy way. A lot of the services are "personal assistant" types, calendar, traffic etc. However it's not designed to be a personal assistant. In some ways it's more than Siri and in others it's less. You can't directly compare the products because their end goals for the consumer are different.
