Providing a full range of features to compete head-on with Skype, Gizmo is making a serious attempt to go toe-to-toe with the current king of free talk. Though it does not yet have the adoption of Skype (both user-wise and device-wise) Gizmo's free voice mail and $0.25 worth of free CallOut credits for new accounts will hopefully attract formerly-hesitant users to the world of VoIP and even win over Skype loyalists.
Gizmo's use of open standards also allows users to take advantage of SIP peering arrangements to expand the possibilities for free in and outbound calling. And Gizmo users can also exchange messages with virtually any other IM service via Jabber gateways. Finally, while Gizmo doesn't show serious signs of wooing business users, its use of SIP makes it friendlier to corporate networks, many of which are already set up to manage SIP traffic.
But while Gizmo has some impressive advantages, Skype's installed base represents a formidable, but not impossible challenge. Skype's proprietary protocol has caused some corporations to ban Skype traffic from their networks, for fear of possible security breaches through Skype's encrypted and un-auditable protocol. It also precludes VoIP peering arrangements, but just as importantly means that Skype users can't exchange instant messages with other services such as AIM, ICQ, Yahoo, etc. (and vice-versa). The other question is whether eBay's purchase of Skype will move it away from general-purpose messaging and more toward support of the eBay ecosystem.
That being said, the choice between Skype and Gizmo ultimately boils down to one of personal preference. Skype's already-enormous userbase somewhat counterbalances the closed nature of its protocol, and the fact that it comes with multiple language packs and a greater variety of international calling plans makes it a much better choice for those who live outside the United States or who plan to travel. It is also a much more 'social' program, offering extras such as 'SkypeMe' (a feature that enables users not on your friends list to call you), custom icons and ringtones, and even a free Skype session with Brit-rock superstars Coldplay.
Gizmo has managed to make some noise and gain some ground against the seemingly ubiquitous Skype. But it faces a long uphill battle before it can knock Skype off the top of the free Internet talk hill.