Unfortunately, for just about any trip we could think up, the driving times were always dramatically shorter than the transit times, so this isn't exactly encouraging people to give up their cars. Le sigh.
Two features that Google Transit doesn't have: route overlays, so you can see what lines are close to you; and NextBus data. The trip planning is all based on schedules; and as we know, Muni has a very casual relationship with timeliness. But interestingly, according to Muni, that's not their fault -- it's Google's. Google Transit uses a very specific framework for transit data, and they don't support real-time vehicle tracking or route overlays.
But this is still a great way to figure out how to get from here to there -- for example, just type "haight and stanyan to el rio taqueria" in a regular old Google search, and ta-da! You're just a 40-minute ride away from meeting up with your friends (versus 13 if you had a car). Now, remind us again why we're still paying millions of dollars in bridge tolls for 511.org's awful trip planner?