I listened to them on a sound board in a shop while I was in the U.S.A. ("tweeters" electronics) about 6 months ago... I was comparing them to the Z6s, and a set of Polk SRs.
The JLs were probably the liveliest of the bunch, and I remember being quite impressed by the midbass out of them. One little gripe I had with them however was that they tended to give a kinda sharp sound to the music.. not harsh or overbearing, just sharp. Electronic and heavy metal music lovers would like them but, I ended up choosing the Z6s because they "seemed" to lend a more realistic sound to female vocals, and the Z6s were slighty better with the imaging, which was VERY hard to pick out unless I stood dead centre and used the test CD they had (chesky?)..
One thing about the JLs, they can keep it together through about any type of music, and don't sound like they are straining at all when playing any type of music through them. I remember thinking long and hard before choosing the Z6s over them, and I think it was only because I was going with what I knew was good by past reports other than the sound of them compared alone. The JLs made everything sound tight and vivid, compared to the Z6s. Actually, the JLs may have sounded that way because the were brand new at the time. Crap...They probably would have sounded better than the Z6s after running in...
The Polks... well, they were slightly more expensive than the JLs and the Z6s (the Z6s were on special), but I liked their sound.. very realistic but a little boring to listen to after hearing the JLs at full blast. It was interesting to hear how different each of the splits sounded to each other on the board, I would have thought they would have all sounded much the same being "reference" type splits, but not so.
All the splits were set from the same amp which had them high passed at 80 hz (dunno what slope they were at though).... I would have liked to see how each of the splits would have handled the music without the high pass.....
Just as a further note, I've got slight hearing loss at around 3k-4k (by about 10db) measured last year, so while I thought that the JLs sounded lively, someone else may find them harsh.... I love my Z6s, but I have heard other ppl coment on them being a little too bright at high volumes (various opinons I found on the net).