My trip to Jool (or Laythe, to be correct) was made using the thermonuclear engines, because you have more than enough time to burn, and they have the best fuel efficiency in space. My ship had a science lander module (just a cone with science modules attached, solar panels, batteries, antennas and a parachute) and a relay satellite (solar panels, batteries, antennas and satellite dishes for long range communication to kerbin, RCS). Not much but enough for a good science mission. The big breaking burns took 4 to 6 minutes full-throttle, but you dont even see the fuel bar move. It worked on the second try, but you have to correct your course at several points. like on 1/4, 1/2 and 3/4 of the way to jool. Just watch and correct on and on with maneuver nodes, so you will get into jools area of infuence as close as possible.