Day to day, I get all my weather (current location and upcoming locations based on travel itinerary), my financial interests' status (based on google finance portfolio), traffic (based on calendar event locations).
Occasionally, it really saves my bacon. I was about to head over to JFK in a few hours to board a flight to the west coast when Washington DC suddenly became a massive no-fly zone and dozens of flights got cancelled on the spot[0]. I had no idea, and probably wouldn't have known until I got to the airport a few hours later and reached the gate. Except—Google Now alerted me that my flight was cancelled the moment it happened (even before news broke about what caused it). Immediately we hopped on the phone and got another flight booked, before the airline itself even started rebooking flights. In fact, the airline never even bothered to alert us that the flight was cancelled.
Similar situations have happened with traffic through construction, presumably pulled from Waze alerts ("hey your next appointment is at X, and normally it would take 25min to get there but today it'll take 40min because this freeway is completely closed").
Google Now is basically a collection of a dozen services that I rather not bother manage independently, all without any interaction or signup. Yes, it "reads my email" and my calendar and the queries I make through it.
Another fun example: I was researching which phone I wanted to buy next, so I was googling a bunch of phones that haven't been released yet. Few weeks later, it pops up on Google Now "hey, you searched for this and there's news about it today."
Lots of cute little surprises when you travel, too. Entering a new country? It'll show you the local currency exchange and the timezone difference (and nearby attractions that I often ignore).
Anyways, it's a great integrated experience and just keeps getting better over time.
[0]. http://www.washingtontimes.com/news/2015/aug/15/dc-new-york-...