Are ultrasonic ant repellents effective?
Not sure about how effective they are specifically on ants -- but I do know one major drawback with these devices: they don't work around corners or behind objects.  In other words, they are line of sight only.  If the ants are behind cabinets, appliances, in the walls, etc, these devices won't work.

My own approach to this issue has been to use pesticide in the yard so they don't get into the house in the first place.  That, and I try to make sure the house is clean (floors vacuumed, counters wiped, food in sealed containers, etc).