I've use single and twin jets while helping with testing for my old company. I think they are great, twins are even better than great for manovering and working with ropes etc in the water all around you.
If you search the threads you will find a previous discussion on jets, main point from memory is that if your intact comes out of the water you loose drive and stearage until the water is returned to the system.
Also depending on the drive configuration they can be expensive to run, well ours were but that was down to the choice of parts.
All aside twin jet cat is the most manoverable boat I have ever driven, though getting use to the jet is fun