Om asks the question, Is VoIP an Excuse for Bad Voice Quality?, the answer is no, it is not.
The truth is that VoIP is capable of delivering better quality than most are used to but the technology is not given the chance. Best efforts VoIP from companies like Vonage and alike is cheaper to deploy and less hassle to maintain.
Some companies are getting right. VoIP delivered with stand alone connectivity that is dual redundant and fault tolerant can sometimes rival the experience we are used to from POTS, as Andy points out.
It is likely that companies that leverage VoIP at the core and deliver voice via traditional (and potentially IP) means to the end user will likely do better in the shoft term than pure best efforts VoIP plays. These hybrid service providers will also be positioned well for future growth as the core of the network is already primed for delivering VoIP at the edge of the network.