I have been using VOIP phone in my house for more than 2 months now. There have been no issues yet. I'm using the service from Lingo. This is very nice. I get all the great features of normal phone including Caller-ID, Call-Waiting, Simultaneous Ring, Call forwarding, Voice Mail and many more which can only be offered in a VoIP phone. For example I can set rules that allow me to forward calls to my cell phone when I'm not home (or only during certain time etc.). My voice mail can be automatically forwarded as an mp3 attachment to my email. I will never have to change my phone number again. All I really need is Internet connection. With broadband becoming ubiquitus this should not be a problem in the future.
There are many new things that can be thought when your phone service is delivered over the net: For example why should it not be possible to have a 'Universal Internet Phone Book' that I can synchronize with my Outlook Contacts list? With so many portable devices why should I have to manually maintain multiple copies of my Contacts? I have a cell phone and a PDA. Both have synchronization capability so they can be automatically synced. Why not have similar thing for my VOIP phone?
I'm sure there are many such ideas in the making and we'll see them coming out as the VoIP usage increases.