You can install a preview of Mattermost on a Mac in about 5 minutes with Boot2Docker in a single container instance, instructions here (email not enabled in Docker instance): https://github.com/mattermost/platform/tree/v0.6.0#mac-osx
In terms of messaging functionality, there's nothing ground-breaking about the UI, our focus is on avoiding what happened to SpinPunch when we were previously locked in to a proprietary SaaS messaging service: http://www.mattermost.org/why-we-made-mattermost-an-open-sou...
That said, Mattermost was originally designed to be a games portal (not just a messaging app), and you'll probably see the UI bring that functionality back in the "AppCenter" feature being developed: https://docs.google.com/document/d/1s6vrticxgz9PsxePQ-dFrsKp...