Personally the only long arm kit out there that is worth a shit is the Genright and it is stupid expensive. You are not going to see much gain at all by doing a long arm kit that is offered today because there are too many limitations on the JK without hacking it up and moving the fuel tank on the rear and up front most kits are only a couple inches longer the the factory arms. IF you want a great set up for a daily driver and wheeling look at Metal Cloak or Synergy, they both have options in your lift height and use a good joint that gives superior ride quality vs the ball and socket type. If you don't like those then I would look into genright, EVO, Currie, Clayton, JKS. Learn what's in a lift kit and what you need for the height and tire size you are looking at.
I've been running a Teraflex Prerunner long arm for the last six years. Overall I'm pleased with it, but their upper flex joints are too small for the loads they have to manage and self destruct regularly. The lower flex joints are fine. It spits out a clevite joint occasionally. Teraflex threw out their flex joints and clevites, replaced with the Alpine all-clevite design. Getting very good reviews.
A long arm is marginally more stable on the trail than a short arm, and highway ride is improved. All that said, you can get virtually as much out of a front drop bracket added to a short arm as you do with a long arm. I would not do another long arm on a JK. JL remains to be seen. The two long arm kits I've seen use a much longer rear upper arm, made possible by the JLs wider track.
'09 JKRU. 4" Teraflex long arm, Teraflex R44 housing, Teraflex / Fox Resis, Poly Performance skids, Nemesis Billy Rockers, Nemesis flares, 37" ATZ P3s on 17" 5x5.5" Trailready HDs, Spyntec hubs, Teraflex BBK, Mopar BBK master/booster, Superior front & Alloy USA rear axle shafts, CTMs, Reids, Synergy drag link & tie rod, WTOR hydro-boosted steering, 5.38s, TF Dual Rate front bar, TF bumpers, Warn Powerplant, Tom Woods shafts, AA xfer case cable, Kilby canister relo, RH4x4 cage.