There's some good Christian metal out there. Tourniquet (who not too long ago released a new album) at the top of the list. They're a thrash band I'd put up against Metallica any day (and I used to be a major Metallica mark). Early Deliverance for more thrash, though not as good as Tourniquet. Underoath if you like the heavier stuff. Six Feet Deep if you like a hardcore/groove metal. Even early Whitecross if you happen to like that high-pitched early 80's metal style. The lyrics are all of questionable theological precision, but they're heartfelt, and I'd rather listen to people singing about how they love Christ and are trying to be good Christians than to listen to most of the alternatives.
Oddly enough, I can
think of theological reasons for not listening to metal. More than half a dozen, actually. But it's all subjective. Music that might whip one person into a frenzy (making him want to get into a mosh pit), for another person relieves tension. Music (let's say, classical) that might have a profound spiritual influence on someone, might bore another person and put them to sleep. For my own part, some of the theological reasons against metal are valid in my own life. But having tried to go without such music for well over a year, I can safely say that it's better (for me at least) to get a little dirty with Christian Metal, than to get really muddy without it.
It is better both to attain the good and to keep the purification. But if it be impossible to do both it is surely better to be a little stained with your public affairs than to fall altogether short of grace; just as I think it better to undergo a slight punishment from father or master than to be put out of doors; and to be a little beamed upon than to be left in total darkness. And it is the part of wise men to choose, as in good things the greater and more perfect, so in evils the lesser and lighter. Wherefore do not overmuch dread the purification. For our success is always judged by comparison with our place in life by our just and merciful Judge; and often one who is in public life and has had small success has had a greater reward than one who in the enjoyment of liberty has not completely succeeded; as I think it more marvellous for a man to advance a little in fetters, than for one to run who is not carrying any weight; or to be only a little spattered in walking through mud, than to be perfectly clean when the road is clean. - St. Gregory the Theologian, Oration 40, 19
PS. I should add that Christian Rock isn't that much different than Secular Rock as far as quality. Probably 1% of both secular and Christian bands are really good. The difference is that when it comes to secular, that 1% comes out to like 500 bands in the history of rock, since 50,000 bands have tried. When it comes to Christians, though, maybe only 5,000 bands have tried, so there's maybe 50 good Christian bands. And while there are good secular bands who very few have heard about, because of the nature of the music business, there's even more Christian bands who people have never heard about. Thus, it seems (to those who have never really looked hard) that Christian bands suck. They don't. If someone puts an effort into finding good Christian bands (in every genre), they will find them.