I own them and am a huge fan. For me, they are THE ultimate headphone for rock and metal; no other headphone I've ever bought or auditioned comes close. No, they certainly aren't perfect, and I would actually say they're quite poor "all-arounders" for anyone looking for one reference headphone. My R10s outclass the L3Ks in almost every category (tone, soundstage, refinement, imaging, highs, mids, etc.) but I have yet to hear a headphone provide the same level of weight to the music that the L3Ks do, and the bass is really phenomenal, IMO. To me, what the L3Ks do best is recreate that "front-row at a rock concert" effect, that feeling that you're not that far from the amps and can really feel the THWAK! THUNK! CHUGGACHUGGA! BOOMBOOM! of the drums, guitars, and bass. My two biggest problems with the L3Ks is cleaning up the sound just enough to where it doesn't lose its weight and immediacy, but still allows for greater clarity and maximizing the soundstage and air. Also, one problem I rarely see mentioned is the the L3Ks are rather weak in the treble region. They're rather light (probably where they "dark sound" comments come from) and they just sound really off... I can especially hear this on cymbals which sound overly metallic or even plastic-like.
My idea tube combination for the L3K on my SDS-XLR is the 6BL7GT flat plates as drivers and 5687s as outputs. The former are extremely open and neutral with a huge sounstage. The 5687s have a lot of energy, extremely dynamic but they achieve this by increasing the speed and thinning the sound which provides greater linearity. This is perfect for the L3Ks because now I get an acceptable soundstage, a trimming of their "thickness", and greater clarity. They can still get a bit messy on extremely big, noisy, fast/complex passages (like the new Immortal album I was spinning recently, which, admittedly, it's hard to cut through black metal fog on any system), but for reproducing that "YOU ARE THERE" feeling, they're my favorite headphone.
As for comparisons, I much prefer them to my RS1s and the ED 9s which I sold. The L3Ks are somewhat like a big brother of the Grado, even though the tonal signatures are quite different. The RS1s are faster and leaner, the L3Ks are slower and thicker; I prefer thicker (the L3Ks own the RS1s when it comes to bands like, say, AC/DC where the music doesn't need the RS1s speed). The ED 9s, while they improved with burn-in, never did sound quite right to my ears tonally. They had amazing bass extension and impact, but the bass also screwed up the mids and the soundstage never felt coherent to my ears. I also agree with another poster who said that they're not a big fan of the modern, hi-fi sound that's extremely clinical/detailed, probably because I think my player, cables/rack, and amp provide such a heaping of details that the headphones are needed to add that musicality and flavor.