How reliable is the 911 service on VOIP phones?

I have recently got rid of my Embarq phone service at home and have switched to ITP VOIP which is cheaper and better. However, I have read a lot about how 911 service doesn't always work well with VOIP. Can anybody with a VOIP phone service (eg. Vonage/ITP/Lingo/etc.) who has attempted to call 911 on it before tell me if it actually worked or not?