I use it for a few reasons and not necessarily related to my android phone.
1. The click to call feature. You can install a google voice extension to the google chrome browser. This puts a button in the browser toolbar that drops down to allow for instant dialing or texting. If you select dial, you can input a number manually, or type a name from your google contact list and it will populate the field for you. Then select which phone you want to use to handle the call (from a pre-populated drop down list such as home, office, mobile). When you click connect the chosen phone will ring, and when you answer it, it will then ring the intended target (I'm in sales and do a lot of calling so this cuts down on my manual dialing dramatically)
2. At the core of the GV service it is a forwarding service. So you can have that number route to any or all of your phones. For instance, when I am in my office inbound calls ring my desk line. When I am out in the field with clients, it will forward to my mobile. Or I can have it ring all my numbers all the time.
3. it can completely replace your mobile's voice mail. And the google voice mailbox is pretty robust with features, such as various greetings per caller, email notification of voice mails and voice transcription to email (though this is more often comical than useful)
4. of course the free texting doesn't hurt either. AND you can text right from your desktop using the google voice web interface. The conversation management is pretty cool too.
downsides are
1. it is another new number that you may need to distribute and get people using
2. texting doesn't replace your native SMS, so if people are texting you to both your mobile number and GV number you will have to use 2 apps on the phone to manage the conversations.
