Are JJs worth installing?
Most definitely. Added maneuverability, ability to jump over obstacles (including other mechs), and tactical advantage they can provide is a huge bonus. Most top tier competitive teams have nearly all their mechs fitted with JJs, and the ability to mount JJs is an advantage over similar mech models that don't.
How many should I install?
Lighter mechs can get away with 1 and still gain some increased maneuverability and jumping range. Heavier mechs are no longer as maneuverable in JJs as they once were, and don't get a lot of lift out of 1 JJ.
2 seems to be the best compromise if you don't need outrageous jumping hight. It provides a good solid fast lift for peaking over or maneuvering over obstacles in battle. Be aware, JJ Fuel runs out faster than it used to, so you don't have as much time in the air.
Mounting 3 or more should give you great height and maneuverability. If you need to get into the air on a regular basis, it's actually worth mounting 3 or more JJs in a mech now. There are definite and noticable advantages.
Old answer:
Ultimately, keep in mind that you get diminishing returns from JJs. JJ1 provides some amount of thrust, and JJ2 provides a bit less. Additional JJs do not increase the amount JJ fuel used in a jump (ie, how long you can jump for), but you get more thrust out of using your JJs.
Remember. Everything is a tradeoff. In selecting a mech with JJs, you chose to sacrifice some small part of your tonnage for additional maneuverability.
Edited by Bront, 05 March 2014 - 08:11 AM.