I'm curious to see what other people think of as where JKL is going to fit into the game. Here's my opinions/predictions on how he'll be utilized over the first couple months following his release.
Arena: I'm not so sure we're going to see a ton of him in the arena, or if we do, it'll be as a 5th for GL Rey, rather than as a lead. Sure, he can probably make a Jedi team that can take out DR, but with so many other options that people have available for that I'm not sure anyone who can unlock JKL on round 1 won't have a better option.
Raids: He'll probably perform fairly well here, especially under a JKR lead. I don't expect to see his lead being used, since it can't limit bosses' speed. You'll probably get a minor boost to your HSTR scores, but nothing game changing like with GAS or SLKR.
GAC: Another minor boost. Sure, he'll be able to make your Jedi better than they would be under JKR, but it'll be more of a one or the other, not both, since they're going to require a lot of the same toons.
TB: It seems like this is what JKL was designed for. He'll be a leader that will make a second Jedi team viable for LSTB. I wouldn't be surprised to see him leading GAS, Ahsoka, Hoda +1 very effectively, allowing you to keep your GK/JKA for Padme. Whether or not anyone will have him before they don't need him anymore (due to getting KAM at 7* as another route for a viable Jedi team), only time will tell, I guess.