Do the ultrasound bone healing machines (exogen 4000+) actually help to speed up fracture healing or is it just a gimmick?

Ultrasound. Not a gimmick! although no treatment is infallible, ultrasound treatment of fractures has been shown to help in bone healing. This is mostly used in delayed unions especially with fractures notorious for healing difficulties (like the tibia or the 5th metatarsal base).