Here's what I've heard so far. I'll try to include as many bands, even a couple I don't like, to be fair:
Gackt - J-Rock/J-Pop. As his songs can be diverse, I can only say that his vocal range and strength is extremely fine-tuned. The general atmosphere is mysterious, moody, or fun, or somewhere in between. His instrumentals are wonderful (freesia op. 2; sekirei, instr. ver). Recommended song: seki-rei, birdcage.
L'arc en Ciel - J-Rock/J-Pop. Somewhat more diverse than Gackt, mostly because they have 10 or so albums to his 3 1/2. Can range from bossa nova-ish (nemuri no yosete) to dynamic (finale, forbidden lover) to grunge (Shi no Hai), to pop-rockish (most of the tracks from Clicked Singles Best), and much more. Recommended song: forbidden lover, finale.
Nobuyuki Hirakura - Did you like L'arc en Ciel? Nobuyuki isn't really in a band, but he did do some nice solo acoustics for L'arc en Ciel. Recommended song: forbidden lover.
Hyde - He is the vocalist from L'arc en Ciel. He only has had one full album out (two actually, but the second's the english version of the first), but that album is well heavily orchestrated. I expect he won't stick to one style, however. Recommended song: Shallow Sleep.
Luna Sea - They have three phases: Lunacy, pre-Shine and post-Shine (that's my own unofficial designation). Lunacy (indies period) is very heavy and dark, very angry. Pre-Shine is very innovative but still has the roughness of Lunacy. post-Shine might seem poppish as others have said, but to me it seems more like a more refined rock. In the latter they use more effects. Recommended song: Genesis of the Mind (pre-Shine); Breathe (post-Shine)
Lareine - Romantic rock. You would think it's french in atmosphere. Recommended song: Emeraude
New Sodmy - vocalist the same vocalist as the one for Lareine. Romantic rock. Unfortunately, I believe they've disbanded, but what they have is good. Recommended song - Romantic Season.
Buck Tick - Think Euro-Goth; although I think some might disagree with me. Recommended song: In heaven
FAKE? - The vocalist is half-british, so the english lyrics aren't as accented. Seems very similar to more Western modern rock. Recommended song: Eternal Satellites.
Lastier - I seldom see their music around, which is unfortunate. Most likely disbanded. I always think U2 when I hear them. Recommended song: SkY, for lack of knowing many other songs.
Hope that helped.