If you are joining to view the presentation, you will join instantly! No download. However, if the host is starting a session to view your screen, you will be prompted to download the Glance guest download.
You can see what it's like by joining this live (automated) session from your computer, iPhone, iPad, or Android mobile device: Click here to check out the demo.
Glance Screen Share guests are much more likely to connect to their session than they would to a traditional web conferencing service (like WebEx, Citrix GoToMeeting, Microsoft LiveMeeting, IBM SameTime), because those offerings often depend on just one browser technology or make their participants install software.