Do you want to know if your a co-worker is available for lunch meeting without being a bother? If this person uses Google Calendar as part of Google Apps (and has shared his calendar with you), you can now visually compare his availability with yours. This significantly reduces the amount of time it takes to find "a good time for everyone" when setting up a meeting.
Ian Murdock adds that the new feature does have a flaw though -- it only searches the users main calendar for conflicts. Often people have multiple calendars like "home" and "work" for example. Once that gets patched up, this could become a widely used feature by those using Google Calendar on a day-to-day basis.