[kjs3]

We have bats. IME, they're no big deal outside of the house (cute-ugly, but don't play with them), but if they annoy you putting up some bat houses away from where you don't want them seems to work. Inside your attic, otoh, they're a nightmare. If you suspect they're getting in, fix that ASAP. Most places (that I know of, ymmv) there are licensed pest management folks who can physically remove the bats.

[WalterBright]

Bats keep the mosquitos under control.

[kjs3]

I didn't feel the need to enumerate every advantage bats have. And where I leave, there are not nearly enough bats to keep the mosquitos anything like under control.