In his article “Skip Skype, Vonage: Get VoIP for Next to Nothing” Brett Arends describes how easy it is to setup VoIP on your own and wonders
"how any telephone companies, including voice over Internet providers, are going to make any money at all down the road."
In this case, he must mean by "voice over Internet provider" companies like Vonage because he uses a VoIP service provider account in the DIY setup too (a Gizmoproject/SIPphone account).
It should be noted that Brett's DIY setup is a bit of an apples and oranges comparison with a Vonage service, because Vonage (and all the other companies offering more or less the same thing) offer a full replacement for a landline, including services like forwarding, voicemail, and (in particular) 911. Also, if you are paying Vonage $25 a month, it's more likely that they will take your call if you need technical support, whereas most DIY service providers offer little or no support (ironically, almost none of them have telephone numbers that you can call).
The DIY setup described in the article WILL NOT provide 911 service (or 411, 511 etc.) That's something to be aware of. Depending on the provider, it may or may not include voicemail, but even it it does, it is unlikely to be as seamless or integrated as a Vonage service.
I also think the article overstates how "easy" it is. While the Voxilla configurator is a terrific tool, many (most?) mainstream users would be overwhelmed in trying to repeat the setup Mr. Arends describes. And if anything goes wrong, they will be up a creek.
I'm not suggesting that Vonage (or one of their competitors) is a better choice, I'm simply pointing out that, to be fair, it is not a apples to apples comparison.