It's important to know that Google Now is meant to understand natural language queries. This means you can fire up the microphone (by tapping the button or saying "Google" while your Search app is active) and simply ask the app who won the Lakers game last night or what the weather will be like in New York this weekend. In either case, thanks to Google Now, the Google Search app should understand your question and be able to return an acceptable answer. From the Settings screen, you can elect to have the app respond only with text on the screen, or audibly with speech output.
When using these sorts of natural language queries, the search experience using the app is really impressive. Because the app takes into account things like your location and search history (with permission, of course), it does a really good job at pulling up search results that are most relevant to you.
The other big part of Google Now is its predictive ability. If you give it permission, Google Now can learn from your search patterns, calendar, e-mail, and more, in order to preemptively serve up "cards" with what it deems to be important information. These cards show up on the Search app's main interface, and as of now they come in about two dozen varieties. For instance, if you routinely search for Golden State Warriors information, Google Now will create a card for the latest game scores when they become available. It can also keep you apprised of your upcoming appointments, flight information, traffic, stocks, and more. I searched for winery here in San Francisco, and later noticed a card pop up with directions there from my current location. While I didn't necessarily need that information at that moment, I liked the idea that Google was thinking ahead in that way.
All that said, Google Now's cards do take some getting used to. Because they're a more or less passive experience and you can't program them, they can seem a little random at times. Unavoidably, you will encounter cards that Google thinks are relevant to you, when in fact they aren't. Just the same, you'll probably find yourself wanting a particular card to show up, and it won't.
While my colleague Jason Parker considers Google Now cards "neat" but not particularly useful for iOS, I choose to give more weight to the instances when they do get their predictions right. Also, I am excited by the fact that these "smart" cards only get better the more I use the Google Search app. So, while Google Now may not be integral to the Android experience just yet, I am fully expecting them to reach that level very soon.
As an interface for conducting a Google Search, the official Google Search app for Android is unbeatable. But more than that, the app offers personal assistant features that can understand natural language and predictive features in the form of Google Now cards. These make the app a lot more than just a way to search.