I've been using VoIP for a couple of years and dropped my landline last year but retained my cell phone for emergency purposes and out of town trips. My general impression of VoIP is it is cheap and acceptable way of communication though don't expect landline clarity.
If you are a heavy long distance caller then savings is about 50 percent plus the fact that even if you travel and as long as you can get access to a good connection then then you'll get to save on long distance calls or even overseas calls.
I used quite a few voip providers and my least favorite is Vonage. Sunrocket is very good but was mismanaged by a certain Lisa Hook
I signed up with Teleblend a few days after but they can not get their acts together so I asked for a refund after 3 weeks and I give props to them for making it easy for me to get my money back. Now I am using and trying Onesuite. It is compatible with most ATA devices though don't expect help from them because at the moment they only support SJ labs softphone software. Onesuite being a phonecard and voip is a plus because you can use any of the two services anytime. The phone card service comes handy especially if you are not at home with limited or no access to the internet.