JJ's are good for moving across intervening terrain. You don't need to roll for terrain features if you jump over them after all, and they all cost 1 MP to move across, so broken terrain is no impediment to a mech with JJ. Yes, this means that while a mech would pay double MP to move through some light woods, JJ is just one basic movement point. That Wraith mentioned up above will outrun anything that doesn't have a 14 run speed if it jumps across 7 hexes of woods, and a jumping mech gives a +1 to hit for anyone shooting at it, plus the usual penalties for hex travel. So trying to nail that jumping Wraith is at a +4 for the attacker, which is not good. Jumping also ignores MP costs for upwards level changes, so long as your initial jumping MP after modifiers is equal to or greater than the terrain height, and going down any number of levels is just 1 MP. Turning also costs no movement points, unlike walking. As you can probably imagine, something like a Victor and its 4 hexes of jump allows for some extremely surprise buttsex whenever terrain gets rough. Now, its 4 heat for every jump MP expended, but you don't get penalties until you hit 8, and while you're at a +3 to hit thanks to the jumping, you're hopefully at short range and you've got an AC/20, 2 medium lasers, and an SRM-2, and at least one of those will hit, and not even an Atlas wants a medium laser to light up its rear. Yes, it has 2 ML there, but he's at a +2 to hit from the rear arc plus the +2 to hit thanks to you moving, which is a total of +4, so you've got some very good odds, and unless said Atlas stood still he gets a +1 from walking, so that's a total of +5 to hit for him. Advantage: Victor.
Phew, autism over.