Other than the high price and the fact that I didn't use the service, I had very few complaints with my actual Vonage service. However, I do find it rather annoying that Vonage does not permit its customers to cancel online.
Their website doesn't make it particularly easy to identify even the number to call to cancel (in the US, 866-243-4357). When I called to cancel, the automated attendant asked a number of questions to verify my identify, like asking me for a PIN I never use.
Finally, after successful navigating that, I was able to talk to a person. However, he was really there to sell, so after 15 minutes of repeatedly having to reject marketing pitches, I was finally able to cancel my account.