Another Ooma user here. I used to work for a different VoIP company, so I could explain in excruciating detail why Ooma's combination of hardware, software and protocols and codecs is best, but I'll resist the temptation. The fact that it's cheap is icing on the cake.
One major caveat, though. Any VoIP provider will pale in comparison to analog phone service if it's connected to an Internet connection that is lacking in some ways that don't necessarily show up in other uses of the same connection. (Latency and jitter, for the technically savvy among us.) I pay Time Warner an extra $10 month for their higher bandwidth Internet option because it makes my Ooma service work better. If you cheap out on your Internet service, you'll make ANY Internet phone service sound awful.