The features themselves are not remarkable, but do bring Voice some parity with similar Skype features and add-ons. The extension makes phone numbers on Web sites callable via Google Voice by just clicking on them. You can place calls and send free text messages by typing any number or contact name, and you can view the most recent messages you've received. The toolbar button displays the number of unread messages you've gotten and offers fairly quick access to recent messages and their transcripts.
Most phone numbers are clickable, but not all of them. For those that aren't, which includes numbers in Google Calendar, highlighting a phone number will open the click-to-call pop-up. In the extension's Options menu, you can toggle the clickable numbers option and set which tab the toolbar button defaults to, or deactivate it entirely.
Voicemail playback isn't supported in the current version, although Google says that it's being worked on.