I have tried quite a few, everything from 25hp up (outboards). They are ok but the boat does need to be matched to the jet, some boat work with jets others dont.
The MoD had quite a few 6.5m Delta's with twin Mariner 90hp Jets for a few years, i then went round the country taking them off and putting gearboxes back on.
You do loose quite alot of power with the jet, for example we fitted a 5.4m Searider with a jet outboard but ended up with a 115hp mariner on the back to get near the same performance of a 75hp.
The are also quite bad a getting blocked up, weed is a killer and its not easy to clear the jet/impeller as you have to get your tools out and take the grate off the jet intake.
Not a controlable, like a jet ski you do need a bit of power on to get them to turn!.
We also put a Pair of Optimax 225's on Jets on a Halmatic 28ft rib, they did 600hrs ish, but the mod decided again they were not suitable, due to the loss in performance.