I've seen a lot of hype about VOIP mechanisms in the past! But, after having used Skype for several weeks, I can definitely recommend it as a marketing tool.
It has a vast user base - 150 million downloads, I think - and there's usually about 3 million users online at any one time.
Like any popular system, Skype has attracted a lot of 3rd-party developers, and there's an increasing number of add-ons and plug-ins. This is where the marketing interest lies for me.
Today I installed "Skype Card" on one of my websites - look at the footer on any page on my TKA website The Skype Card allows my website visitors to contact me in a variety of ways (if they have the correct plugins installed). The Card even shows the visitor if I'm online with Skype (again, if they have the plugin installed).
My feeling is that Skype is going to get better and better. If you're not Skyped yet, then get it. And the service is free.