been using vonage for about 8 years. The reliability is strictly dependent on your internet reliability. Vonage themselves is flawless. but if your internet connection is not reliable then neither will your phone service.
One thing though they aint as cheap as they used to be. they offer you a low intro rate for 6 months then you go to the regular rate which seems to increase about every year as well.
We have skype and it works great. Cost me about 8 bucks every 3 months, dirt cheap! It worth having just in case cell phone/towers have issues. I don't use it for the biz often, but for 35 bucks a year, totally worth it.
The company here in ohio i worked for had a Voip phone system that was set up internally. There 5 or 6 phone numbers coming into the system. We also had several phone through out our facilty where we could transfer and park calls. Lots of cool capabilities with that type of system, and was very reliable.