Chad, VoIP is a mature and robust technology... in fact, nearly all phone service in the United States is already being run over VoIP infrastructure on one level or another now. I will also say that VoIP is robust enough for business needs and is used daily in the enterprise sector.
Now, what can make VoIP service terrible is one of two things:
1) your internet provider
2) your VoIP provider
Fortunately, VoIP doesn't require a lot of bandwidth... what matters is latency and noise on your internet connection.
Use this tool to determine if your network connection is up to snuff for quality voice services. If your network connection is fine, you're half-way home.
As to the VoIP provider? You ultimately get what you pay for. Google Voice? MagicJack? Nettalk? Yeah, those are gonna suck from a quality standpoint. But VOIPo? Future Nine? VOIP.ms? CallCentric? You'll be fine.
Here, have a crash course on VoIP services:
http://www.techmeshugana.com/2013/04/voip-and-the-return-of-the-home-phone/You keep your "home phone" and you don't have to up your cell phone usage.
As for the Lumia decision, there's a couple points to go against it:
1) Windows Mobile. If you're wanting to buy a smartphone for added features and functionality with a thriving ecosystem? There's cheaper, with more apps, and less lockdown. If you're wanting navigation (GPS maps), the only way around the massive data sink is to have offline maps. Android and iOS has that, WP doesn't AFAIK.
2) Buying the Lumia 520 GoPhone and taking it to Airvoice isn't going to work from a wireless data standpoint anyway. The phone you'll buy from AT&T will be carrier locked to AT&T. You might be able to pop in an Airvoice SIM card and have calling and SMS text messaging work, but you won't be able to configure data. You also won't have the freedom to go over to a T-Mobile MVNO if so desired. You want to take a smartphone to an MVNO, you
need a carrier unlocked smartphone.
That should get you started on research. You know where to find me if you need me.