The main feature of Google Voice is unifying numerous phones under a single number.
You could, for example, link your cell phone, your home phone, and your work phone, and anytime someone calls your GV number all three will ring (according to the ring-schedule you set when you added the phone; so, you can make it so the work phone only rings when you would actually be at work); whichever you pick up first is the one you use. If you then decide to switch to a different phone, you can press the * button, your other phones will ring, and then you pick up the one you want to switch to.
You can use the number for call screening (when you answer an incoming call, you'll hear the name of the caller, and you can either answer by pressing 1, or sending them to voice mail by pressing 2 or hanging up; if you press 2, you can listen to the recording, and then answer at anytime). You can then manage your voicemail via either a browser or the app; the messages are automatically transcribed (albeit sometimes these transcriptions are humorously bad), and can be automatically forwarded to your email or texted if you want. Oh, and you can set up different voicemail messages for different groups of people (and it looks like you can set which phones will ring for these groups).
You can send up to 100 sms text messages per day for free (I don't think there's a limit to how many you can receive), from either your cellphone or your computer (which I find much more convenient than even the keyboards on my slider phones). Please note that multimedia messages are not currently supported (unfortunately, it doesn't look like they ever will be).
You can "flick a switch" to go into Do Not Disturb mode for a set time, so you don't receive calls or texts during this time. You can also easily block numbers (phone stalkers? Not anymore! ...Hopefully.)
If someone happens to call your phone's actual number instead of your GV number, and you don't answer, you can set whether they're immediately sent to voicemail, or your other phones ring first.
If someone calls you on your GV number, you can press 4 during the call to record it (this will play a tone and message letting both parties know that the call is being recorded).
I think that covers the cool parts of GV (obviously, using your GV number exclusively results in more cool things you can do).
The bad news is that there is sometimes audio delay. And sometimes it can be pretty bad (with both parties accidentally cutting each other off; you can get used to the lag, and if you let them know about it, it can be manageable). I suggest you get a friend to call you on your GV number, and see how bad the lag actually is, and if it's something you mind, before "plunging in."
