1) You can use E911 with Vonage. You register your number through them and you will always be identified on 911 calls after that.
2) If your cable service goes out, you can have a number as back-up for free. It will auto-forward to that number when either Vonage itself is down or the cable is out. I have yet to see Vonage down... just my cable.
3) My phone calls tend to be clearer than the Googletalk, Voice IM type services.
4) You can do some interesting stuff with Vonage... such as setting up an extra phone number anywhere they have service for only an extra $4.99 a month. It will ring to you, but it could be a phone number in the middle of Alaska. This could be good if you want to make a local phone number for your family who all live in another city or something along those lines where you can make it cheap for them to avoid long distance. You can forward your phone calls anywhere without long distance charges. You can transfer a call anywhere. You can log in online and check phone logs, play voicemails, change forwarding, or do almost anything else to effect your phone line. Changing numbers is simple, if you ever need that.
5) you can also set-up really cheap 1-800 services and other stuff.