Placebo plays a kind of simple but unique alternative rock with quite a lot of influences, from David Bowie to King Crimson to Faith No More. I find them pretty creative, though of Molko's peculiar vocals and their simplistic song structures at times, but still their sound doesn't fit a category (that's why it is labeled as "alternative rock") and their themes can get pretty emotional. I think of it as a great band.
and again, Opeth and Placebo cannot be compared by any means. both are arty, but it's like trying to say "apples taste better than bananas." they're two different sounds, and I indentify myself more with Placebo.