Got to see the bats leave the tunnel and there are more than a few of them.  They say its about 3 million bats right now that take a 30-45 minutes to exit every night.  Got a seat at the lower viewing deck.  It's worth getting out there if you have the chance. 

The park is very small but not a bad drive from either Austin or San Antonio.  If you call they have a recording to say when the bats are typically flying that week.