They are a pile of dung! We installed one on a 1k h.p. cat and what a nightmare! We first had to raise the engine 4" because it is a surface drive which meant reconstructing the back of the boat. Additionally, the good props are around 3k. In addition to the props, custom steering had to be made to fit it. It is a wierd size and standard external steering would not work. It requires an additional transmission and will eat up around 100+ h.p. from the flywheel to the prop.
Once we got all of the parts installed and working right, the owner popped 3 of the drives in about 2 months. Had in total about 15 hours on each drive before they broke. Owner is running a B-max with more luck than the Sternpower.