My hotel was at the site of the ""bats"" so I went down and joined approximately 100 people (young, old, tourists, locals) We waited in a nice park and waited and waited and then out they came. A zillions little bats just kept coming out from under this bridge. I was very weird but cool. I'm glad I saw this because I could never have imagined what 1.5 million bats look like. I also never thought of bats as being ""good"" for society. We could use some mosquito eating bats in Miami. Wonder if they each roaches and toads.... I would plan it into your schedule. Even just being there doing nothing for 15 minutes before dusk is relaxing. If you have on comfortable clothes you can go for a nice walk or run on the lake (that looks like a river).
Pros: unique, unexpected, safe
Cons: quick, bad time of evening