Your friends don't need the application to receive your messages, so you won't have to send an annoying "Add this app" blast to your friend list. While Bobsled is a T-Mobile product, it isn't exclusive to T-Mobile customers. In addition to voice calls, and voice messages, Bobsled is also integrated with Facebook Chat, so you can easily connect with friends with one click.
You're probably wondering why T-Mobile would launch such a service for free. At the moment, it's more of a combined pilot program and promotional tool. As their press release states, "Bobsled by T-Mobile positions T-Mobile as a provider of cloud-based communications services over the Internet."
Additionally, "Bobsled by T-Mobile also powers the Group Text and Cloud Text applications on the new T-Mobile Sidekick 4G." So while they're offering free VoIP calls to 500 million users, they're also promoting their devices and network in a very savvy manner.
The future of Bobsled includes video chatting, and calls to both landlines and mobile devices. They also hope to adapt apps for various smartphones and operating systems, "regardless of the carrier that powers such devices". Move over, Skype.