I've owned allot of JBLs and am probably a bit of a fan boy, L100, Olympus, L77, L88s, 4315s, Decade L26, and a few others. The build quality has always drawn me to them. Massive Voice Coils, Cast Frames, Heavy braced cabinets, to name a few. I think JBL's big thing are there ability to articulate the signal accurately, especially in the bass region where most speakers fail, and there efficiency. I think because JBL speaker were born out of the studio environment these aspects were a big part of there development.
So far I've held on to my Olympus and 4315s and have no plans on ever selling them. That being said, I also own a pair of Bozaks Urbans and my own home built speakers that were built with the old Rectilinear III sound in mind. I use the 4315s in my amateur recording studio and really appreciate there accuracy but for musicality my home built speakers and Bozaks tend to me more musical and forgiving wich is better for everyday listening, especially if the source is not the greatest. The olympus are fantastic for just blowing your socks off and pissing off the neighbors

Not many other speakers can truly represent the sound of Mickey Hart's 23hz Helmholtz Resonator on Plannet Drum