As you've probably noticed, Samuel Vinson has finally released Siphon for the 2.1-2.2 firmware.
The App Store wouldn't accept it, so he released it for free for the moment.
It's not designed to run in the background (though I guess with Backgrounder it could).
Advantages seem to be a user interface that is an almost exact copy of the Phone application, and a direct connection to the server (in contrast to Fring, which connects through Fring's server).
What do you think, how does it measure up to Fring? Has the sound quality finally become acceptable? Was it worth the 13-month wait?