It truely depends on what sound you are looking for. I have been playing fetless for some time now, and I use it exclusively, for all different genres of music.
Fretless *typically* has a much warmer tone than a fretted, and tends to have a *buzz* to the notes, almost like a natural distortion if you will. It is also a much more demanding instrument to play, due largely to the fact that it is played more by feel and ear than sight and touch. Accurate placement of fingers is no longer as crucial, as is your ability to bring a given note into tune before anyone notices. You also tend to use a lot more slides in your playing... giving it a kind of warm, lazy feel.
Your ears then become the most important part to your playing, and you will find yourself defending them in the same way you would your hands. I found myself listening to music differently, and I am now much better at picking out notes by ear. I used to have to look at my hands when I played fretted, and I realized when I picked up frettless that it was going to be of no help.
Bottom line, it all comes down to what sound you want, and how hard you are willing to work to achieve it. With enough practice, and familiarity with your specific instrument you *can* fool people into thinking it is fretted, [but] why would you want too?
One more thing, less blisters: using flatwound or nylon wrapped strings is quite silky on the fingers, and is the only way to go, unless you want to sand your frettboard every week!
If you have any more questions, I'll assist the best I can.
To hear examples of fretless bass with metal music, and performed quite amazingly, check out Ahgora, and Cynic (the album Focus is all fretless bass. And Sean Reinert is one kick ass drummer

)
Good luck!