Been waiting for this feature for some time now. If you travel often, are stuck without an internet connection, but still want to sort through and read your gmail, and don't want to use a 3rd party program like Microsoft Outlook, Mozilla Thunderbird, or Apple Mail, then you'll love this.
You get the convenience of those programs without having to use a separate program to access your gmail without an internet connection. If you have a gmail account, you'll see the feature show up in "gmail labs" (the little green icon in the top-right corner of your gmail screen) section in the next few days. On second thought, Gmail just got more addicting -- damn.