I've been using Drovio (formerly UseTogether) for a few years, mostly on inertia, as they were the "best" I found when looking originally. It suits my needs, but feels a little kludgy at times.
One of the killer features, imo, is that Drovio can be run from the browser, at least for the clients. This lets you get a teammate pairing with you super fast, without having to make them download/install something.
As far as I can tell, so far, Multi doesn't have that ability