Ask HN: Donation button: how much to expect?

Managing an online service with a good little number of users who are pretty happy with the service, I am considering adding a "Donation" button to partially cover my expenses.

Since this is some extra work to implement, and this can provide a deceptive experience, I wonder if it is worth it.

Has anyone implemented this before?

How much money / user did you receive?

Would you do it again?

How does it compares with ads revenue?