It works and its free

You get a free new phone number. This could be used either if you want to integrate several phone numbers into one number, or more likely if you want a spare number so that you dont have to give your actual mobile number to everybody you meet. You could use your Google voice number for personal contacts and receive them on your business phone, or vice versa.
You can send and receive calls from all your mobile numbers and your landline. And you can send and receive texts from/to all your mobile numbers. As another reviewer said, welcome to 2008. You cant send picture messages. If somebody sends you an MMS message it will be sent to your email address(es). And as far as I can figure out you cant then reply from your email address. You can also send and receive SMS text messages from any computer. This is great if you are traveling internationally and want to avoid expensive text roaming charges, or if youve lost your mobile phone or it needs recharging.

Theres a pretty good write up of the features, benefits and limitations on Wikipedia. I signed up for Google Voice soon after they launched it after acquiring it from GrandCentral. As far as I can tell from the revision history, since the acquisition, they havent done very much too The in the face except fix bugs and supports new versions of the iPhone. Im a bit cynical as to why they acquired it from Grand Central and then didnt do anything to develop it in the last 7 years. But my phone number still works, and its free. Wikipedia also recounts the dispute they had with Apple. Apparently it is now integrated with Google Hangouts but I havent tried this. It is mainly intended for US based users, but see Wikipedia for options for international users.

senuti42 about Google Voice, v1.11