I don't know man, I started with FL7 just to toy around and learn about music production. But it was kinda bitchy to actually record stuff, it wasn't optimized for that at all.
Now, I know they have tried making it more like a normal studio software that can record audio and put it out into tracks, but it is still too messy in my opinion. One thing I do love though is the routing in the mixer. You can route anything, anywhere, however you like! FL is truly godlike when it comes to technical stuff like that, you can control stuff by typing in your own mathematical expressions and shit, I don't know if you can do that in any other program?
These days I'm using Ableton Live though, got an LE version of it with my soundcard and then I picked up Live 7 a month after because I fell in love with the amazing speed that you can get stuff done in there. The interface is simple and clean and inspires me to do stuff, unlike f.ex Cubase
But Live does not even come close to the amazing technical stuff you can do in FL but I still prefer Live... you can always do what you want in some way or another.
But anyway, back to your question, I think you could use FL8 to produce metal, why not? You're just gonna have to get used to a different way of working. There are no normal audio tracks in FL8. Instead, everything you record comes up as clips and you have to assign clips to a mixer track. This is a very good thing actually, gives so much freedom into grouping stuff etc, but it takes a while to get used to. You have to develop your own method of work in it.
