Google Now has been integrated into the Google Search App, and provides several features that make Siri look less like a savvy personal assistant, and more like a simple voice-controlled search function. To get the most from Google Now, you should use other Google applications, especially Gmail and Google Calendar. (You should use these anyway -- if you aren't using as many Google apps as you can find, you're missing out on some incredibly useful features.) Google Now integrates with these other applications and proactively gives you important information.
Google Now furnishes the kind of information that is pretty standard, such as constant updates on weather conditions and traffic patterns in your area. But what makes it really useful for lawyers (and other professionals) is the information that it proactively provides. For example, if you have a hearing or meeting on your calendar, Google Now will find the route to that location from your current location, calculate the travel time (taking into consideration available information about traffic delays), and remind you when it is time to leave. It has done this for me on a few occasions even when I have not entered the address of the meeting (though you really should always do this). If you have a flight itinerary in your email (and most airlines email you one these days), Google Now will access flight information and let you know if it is on-time or delayed, and when you should leave for the airport. It also appears to know whether you need to be on the departing flight, or at the airport to pick up a passenger on the arriving flight. (It looks like this may only be currently available for certain airlines, but I'm sure that others will climb on the bandwagon soon.)
Thus, Google Now really functions as the personal assistant that some hoped Siri would be. While some have remarked that this is really just creepy, and worry that Google is further intruding into our private lives, I have found it to be valuable. For starters, Google already has "access" to your information anyway, in the same sense that any email, calendaring, or contact-management application does. The fact that Google Now integrates these various applications with a useful personal assistant does not change (or increase) this access. Even if you already have a top-notch administrative assistant/paralegal/legal secretary/other helper, having an automated assistant like Google Now is invaluable, as it frees that person from reminding you to leave for your deposition, so he or she can focus on important things like drafting your disclosure statement.